Allow credit invoices

[ValentinMezev]

The library currently allows only positive amounts to be reported.
This change allows negative amounts to be reported as well to represent a credit to an existing invoice.
Please, let me know if crediting works differently according to the Croatian tax authority system.

[arvvoid]

Hello,

You are correct; the previous restriction was an oversight. Invoices should indeed allow for negative values to represent credits. Thank you for pointing this out.

I have updated the CI action configuration. Could you please merge the latest changes from the upstream main branch into your PR? This will ensure that the CI environment variables are correctly applied, allowing the checks to succeed.

Thank you again for your contribution!

As for the Croatian tax authority requirements, at the time of writing, I am certain that crediting invoices using negative values (to represent credit notes or invoice corrections) is both allowed and the standard method of handling such adjustments. This is not legal or tax advice; for official confirmation, please consult a qualified tax advisor or contact the Croatian tax authorities directly.

[ValentinMezev]

Hello,

You are correct; the previous restriction was an oversight. Invoices should indeed allow for negative values to represent credits. Thank you for pointing this out.

I have updated the CI action configuration. Could you please merge the latest changes from the upstream main branch into your PR? This will ensure that the CI environment variables are correctly applied, allowing the checks to succeed.

Thank you again for your contribution!

As for the Croatian tax authority requirements, at the time of writing, I am certain that crediting invoices using negative values (to represent credit notes or invoice corrections) is both allowed and the standard method of handling such adjustments. This is not legal or tax advice; for official confirmation, please consult a qualified tax advisor or contact the Croatian tax authorities directly.

Thank you for your response @arvvoid
I rebased and all CI checks are passing. Can you please merge it if you think the change is ok (I don't seem to be allowed to merge the PR).

[arvvoid]

@ValentinMezev merged, thanks! :)