diff --git a/README.md b/README.md
index df0e3fc..51052d5 100644
--- a/README.md
+++ b/README.md
@@ -29,9 +29,9 @@ This library works as a toolkit so you can serve and make your LNURL requests wi
 - [LUD-18](https://github.com/lnurl/luds/blob/luds/18.md): 🆘 core 🆘 client 🆘 server 🆘 tests
 - [LUD-19](https://github.com/lnurl/luds/blob/luds/19.md): 🆘 core 🆘 client 🆘 server 🆘 tests
 - [LUD-20](https://github.com/lnurl/luds/blob/luds/20.md): ✅ core ✅ client ✅ server ⚠️ tests
-- [LUD-21 *proposal*](https://github.com/lnurl/luds/blob/8580e3c8cbfd8fc95a6c0e5f7fcb5b048a0d5b61/21.md): ⚠️ core 🆘 client 🆘 server 🆘 tests
+- [LUD-21 *proposal*](https://github.com/lnurl/luds/blob/8580e3c8cbfd8fc95a6c0e5f7fcb5b048a0d5b61/21.md): ⚠️ core ⚠️ client ⚠️ server 🆘 tests
 
-- ###### Soon. ™
+###### Soon. ™
 
 ## Future work
 - Remove SOS signs from above list (by just working on it)
diff --git a/src/client.rs b/src/client.rs
index 7851323..4417618 100644
--- a/src/client.rs
+++ b/src/client.rs
@@ -133,10 +133,10 @@ impl Pay<'_> {
     /// Returns errors on network or deserialization failures.
     pub async fn invoice(
         &self,
-        millisatoshis: u64,
+        amount: &crate::pay::Amount,
         comment: Option<&str>,
     ) -> Result<crate::pay::client::CallbackResponse, &'static str> {
-        let callback = self.core.invoice(millisatoshis, comment);
+        let callback = self.core.invoice(amount, comment);
 
         let response = self
             .client
diff --git a/src/core/pay.rs b/src/core/pay.rs
index 8939902..2073aae 100644
--- a/src/core/pay.rs
+++ b/src/core/pay.rs
@@ -2,6 +2,12 @@ pub const TAG: &str = "payRequest";
 pub mod client;
 pub mod server;
 
+#[derive(Clone, Debug)]
+pub enum Amount {
+    Millisatoshis(u64),
+    Currency(String, u64),
+}
+
 #[derive(Clone, Debug)]
 pub struct Currency {
     pub code: String,
@@ -26,9 +32,31 @@ mod serde {
         pub convertible: bool,
     }
 
-    #[derive(Deserialize, Serialize)]
-    pub(super) struct Callback<'a> {
-        pub comment: Option<&'a str>,
-        pub amount: u64,
+    pub(super) mod amount {
+        use serde::{Deserialize, Deserializer, Serialize, Serializer};
+
+        pub fn serialize<S: Serializer>(
+            a: &super::super::Amount,
+            serializer: S,
+        ) -> Result<S::Ok, S::Error> {
+            match a {
+                crate::pay::Amount::Millisatoshis(a) => (*a).serialize(serializer),
+                crate::pay::Amount::Currency(c, a) => format!("{a}.{c}").serialize(serializer),
+            }
+        }
+
+        pub fn deserialize<'a, D: Deserializer<'a>>(
+            deserializer: D,
+        ) -> Result<super::super::Amount, D::Error> {
+            let s = <&str>::deserialize(deserializer)?;
+
+            let (amount, currency) = s.split_once('.').map_or((s, None), |(a, c)| (a, Some(c)));
+            let amount = amount.parse::<u64>().map_err(serde::de::Error::custom)?;
+
+            Ok(match currency {
+                Some(currency) => super::super::Amount::Currency(String::from(currency), amount),
+                None => super::super::Amount::Millisatoshis(amount),
+            })
+        }
     }
 }
diff --git a/src/core/pay/client.rs b/src/core/pay/client.rs
index 4926e37..53454f0 100644
--- a/src/core/pay/client.rs
+++ b/src/core/pay/client.rs
@@ -107,10 +107,14 @@ impl TryFrom<&[u8]> for Entrypoint {
 
 impl Entrypoint {
     #[must_use]
-    pub fn invoice<'a>(&'a self, millisatoshis: u64, comment: Option<&'a str>) -> Callback<'a> {
+    pub fn invoice<'a>(
+        &'a self,
+        amount: &'a super::Amount,
+        comment: Option<&'a str>,
+    ) -> Callback<'a> {
         Callback {
             url: &self.callback,
-            millisatoshis,
+            amount,
             comment,
         }
     }
@@ -119,14 +123,14 @@ impl Entrypoint {
 pub struct Callback<'a> {
     pub url: &'a url::Url,
     pub comment: Option<&'a str>,
-    pub millisatoshis: u64,
+    pub amount: &'a super::Amount,
 }
 
 impl std::fmt::Display for Callback<'_> {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        let query = super::serde::Callback {
+        let query = ser::Callback {
             comment: self.comment,
-            amount: self.millisatoshis,
+            amount: self.amount,
         };
 
         let querystr = serde_urlencoded::to_string(query).map_err(|_| std::fmt::Error)?;
@@ -174,6 +178,17 @@ impl std::str::FromStr for CallbackResponse {
     }
 }
 
+mod ser {
+    use serde::Serialize;
+
+    #[derive(Serialize)]
+    pub(super) struct Callback<'a> {
+        pub comment: Option<&'a str>,
+        #[serde(with = "super::super::serde::amount")]
+        pub amount: &'a super::super::Amount,
+    }
+}
+
 mod de {
     use super::super::serde::Currency;
     use serde::Deserialize;
@@ -360,7 +375,9 @@ mod tests {
         let parsed: super::Entrypoint = input.as_bytes().try_into().expect("parse");
 
         assert_eq!(
-            parsed.invoice(314, None).to_string(),
+            parsed
+                .invoice(&super::super::Amount::Millisatoshis(314), None)
+                .to_string(),
             "https://yuri/?o=callback&amount=314"
         );
     }
@@ -377,11 +394,38 @@ mod tests {
         let parsed: super::Entrypoint = input.as_bytes().try_into().expect("parse");
 
         assert_eq!(
-            parsed.invoice(314, Some("comentario")).to_string(),
+            parsed
+                .invoice(
+                    &super::super::Amount::Millisatoshis(314),
+                    Some("comentario")
+                )
+                .to_string(),
             "https://yuri/?o=callback&comment=comentario&amount=314"
         );
     }
 
+    #[test]
+    fn callback_render_currency() {
+        let input = r#"{
+            "metadata": "[[\"text/plain\", \"boneco do steve magal\"]]",
+            "callback": "https://yuri?o=callback",
+            "maxSendable": 315,
+            "minSendable": 314
+        }"#;
+
+        let parsed: super::Entrypoint = input.as_bytes().try_into().expect("parse");
+
+        assert_eq!(
+            parsed
+                .invoice(
+                    &super::super::Amount::Currency(String::from("BRL"), 314),
+                    None
+                )
+                .to_string(),
+            "https://yuri/?o=callback&amount=314.BRL"
+        );
+    }
+
     #[test]
     fn callback_response_parse_base() {
         let input = r#"{ "pr": "pierre" }"#;
diff --git a/src/core/pay/server.rs b/src/core/pay/server.rs
index 16dade5..e1d65c9 100644
--- a/src/core/pay/server.rs
+++ b/src/core/pay/server.rs
@@ -67,7 +67,7 @@ impl TryFrom<Entrypoint> for Vec<u8> {
 }
 
 pub struct Callback {
-    pub millisatoshis: u64,
+    pub amount: super::Amount,
     pub comment: Option<String>,
 }
 
@@ -75,10 +75,10 @@ impl<'a> TryFrom<&'a str> for Callback {
     type Error = &'static str;
 
     fn try_from(s: &'a str) -> Result<Self, Self::Error> {
-        serde_urlencoded::from_str::<super::serde::Callback>(s)
+        serde_urlencoded::from_str::<de::Callback>(s)
             .map_err(|_| "deserialize failed")
             .map(|query| Callback {
-                millisatoshis: query.amount,
+                amount: query.amount,
                 comment: query.comment.map(String::from),
             })
     }
@@ -157,6 +157,17 @@ mod ser {
     }
 }
 
+mod de {
+    use serde::Deserialize;
+
+    #[derive(Deserialize)]
+    pub(super) struct Callback<'a> {
+        pub comment: Option<&'a str>,
+        #[serde(with = "super::super::serde::amount")]
+        pub amount: super::super::Amount,
+    }
+}
+
 #[cfg(test)]
 mod tests {
     #[test]
@@ -335,7 +346,10 @@ mod tests {
         let input = "amount=314";
         let parsed: super::Callback = input.try_into().expect("parse");
 
-        assert_eq!(parsed.millisatoshis, 314);
+        assert!(matches!(
+            parsed.amount,
+            super::super::Amount::Millisatoshis(314)
+        ));
         assert!(parsed.comment.is_none());
     }
 
@@ -344,10 +358,25 @@ mod tests {
         let input = "amount=314&comment=comentario";
         let parsed: super::Callback = input.try_into().expect("parse");
 
-        assert_eq!(parsed.millisatoshis, 314);
+        assert!(matches!(
+            parsed.amount,
+            super::super::Amount::Millisatoshis(314)
+        ));
         assert_eq!(parsed.comment.unwrap(), "comentario");
     }
 
+    #[test]
+    fn callback_parse_currency() {
+        let input = "amount=314.BRL";
+        let parsed: super::Callback = input.try_into().expect("parse");
+
+        assert!(matches!(
+            parsed.amount,
+            super::super::Amount::Currency(c, 314) if c == "BRL"
+        ));
+        assert!(parsed.comment.is_none());
+    }
+
     #[test]
     fn callback_response_render_base() {
         let input = super::CallbackResponse {
diff --git a/tests/lud06.rs b/tests/lud06.rs
index 9574870..91cbd94 100644
--- a/tests/lud06.rs
+++ b/tests/lud06.rs
@@ -31,7 +31,7 @@ async fn test() {
             },
             |req: lnurlkit::pay::server::Callback| async move {
                 Ok(lnurlkit::pay::server::CallbackResponse {
-                    pr: format!("pierre:{}", req.millisatoshis),
+                    pr: format!("pierre:{:?}", req.amount),
                     disposable: false,
                     success_action: None,
                 })
@@ -65,7 +65,10 @@ async fn test() {
         "the destroyer of worlds"
     );
 
-    let invoice = pr.invoice(314, Some("comment")).await.expect("callback");
+    let invoice = pr
+        .invoice(&lnurlkit::pay::Amount::Millisatoshis(314), Some("comment"))
+        .await
+        .expect("callback");
 
-    assert_eq!(&invoice.pr as &str, "pierre:314");
+    assert_eq!(&invoice.pr as &str, "pierre:Millisatoshis(314)");
 }
diff --git a/tests/lud09.rs b/tests/lud09.rs
index 97a6cbc..8464449 100644
--- a/tests/lud09.rs
+++ b/tests/lud09.rs
@@ -33,9 +33,10 @@ async fn test() {
                 Ok(lnurlkit::pay::server::CallbackResponse {
                     pr: String::new(),
                     disposable: false,
-                    success_action: if req.millisatoshis == 0 {
+                    success_action: if matches!(req.amount, lnurlkit::pay::Amount::Millisatoshis(0))
+                    {
                         None
-                    } else if req.millisatoshis == 1 {
+                    } else if matches!(req.amount, lnurlkit::pay::Amount::Millisatoshis(1)) {
                         Some(lnurlkit::pay::server::SuccessAction::Message(
                             req.comment.map(|a| a.to_string()).unwrap_or_default(),
                         ))
@@ -68,11 +69,17 @@ async fn test() {
         panic!("not pay request");
     };
 
-    let invoice = pr.invoice(0, None).await.expect("callback");
+    let invoice = pr
+        .invoice(&lnurlkit::pay::Amount::Millisatoshis(0), None)
+        .await
+        .expect("callback");
 
     assert!(invoice.success_action.is_none());
 
-    let invoice = pr.invoice(1, Some("mensagem")).await.expect("callback");
+    let invoice = pr
+        .invoice(&lnurlkit::pay::Amount::Millisatoshis(1), Some("mensagem"))
+        .await
+        .expect("callback");
 
     let Some(lnurlkit::pay::client::SuccessAction::Message(m)) = invoice.success_action else {
         panic!("bad success action");
@@ -80,7 +87,10 @@ async fn test() {
 
     assert_eq!(&m as &str, "mensagem");
 
-    let invoice = pr.invoice(2, Some("descricao")).await.expect("callback");
+    let invoice = pr
+        .invoice(&lnurlkit::pay::Amount::Millisatoshis(2), Some("descricao"))
+        .await
+        .expect("callback");
 
     let Some(lnurlkit::pay::client::SuccessAction::Url(u, d)) = invoice.success_action else {
         panic!("bad success action");
diff --git a/tests/lud11.rs b/tests/lud11.rs
index 8f92163..49e698a 100644
--- a/tests/lud11.rs
+++ b/tests/lud11.rs
@@ -32,7 +32,7 @@ async fn test() {
             |req: lnurlkit::pay::server::Callback| async move {
                 Ok(lnurlkit::pay::server::CallbackResponse {
                     pr: String::new(),
-                    disposable: req.millisatoshis % 2 == 0,
+                    disposable: matches!(req.amount, lnurlkit::pay::Amount::Millisatoshis(a) if a % 2 == 0),
                     success_action: None,
                 })
             },
@@ -57,10 +57,16 @@ async fn test() {
         panic!("not pay request");
     };
 
-    let invoice = pr.invoice(314, None).await.expect("callback");
+    let invoice = pr
+        .invoice(&lnurlkit::pay::Amount::Millisatoshis(314), None)
+        .await
+        .expect("callback");
 
     assert!(invoice.disposable);
 
-    let invoice = pr.invoice(315, None).await.expect("callback");
+    let invoice = pr
+        .invoice(&lnurlkit::pay::Amount::Millisatoshis(315), None)
+        .await
+        .expect("callback");
     assert!(!invoice.disposable);
 }
diff --git a/tests/lud12.rs b/tests/lud12.rs
index 78755d2..ac57dcc 100644
--- a/tests/lud12.rs
+++ b/tests/lud12.rs
@@ -59,11 +59,20 @@ async fn test() {
 
     assert_eq!(pr.core.comment_size.unwrap(), 140);
 
-    let invoice = pr.invoice(314, None).await.expect("callback");
+    let invoice = pr
+        .invoice(&lnurlkit::pay::Amount::Millisatoshis(314), None)
+        .await
+        .expect("callback");
 
     assert_eq!(&invoice.pr as &str, "pierre:None");
 
-    let invoice = pr.invoice(314, Some("comentario")).await.expect("callback");
+    let invoice = pr
+        .invoice(
+            &lnurlkit::pay::Amount::Millisatoshis(314),
+            Some("comentario"),
+        )
+        .await
+        .expect("callback");
 
     assert_eq!(&invoice.pr as &str, "pierre:Some(\"comentario\")");
 }
diff --git a/tests/lud16.rs b/tests/lud16.rs
index cff5773..2bbb660 100644
--- a/tests/lud16.rs
+++ b/tests/lud16.rs
@@ -28,9 +28,9 @@ async fn test() {
                     })
                 }
             },
-            |req: lnurlkit::pay::server::Callback| async move {
+            |_: lnurlkit::pay::server::Callback| async move {
                 Ok(lnurlkit::pay::server::CallbackResponse {
-                    pr: format!("pierre:{}", req.millisatoshis),
+                    pr: String::from("pierre"),
                     disposable: false,
                     success_action: None,
                 })