Request for mapping to BT-120

[diedthunder]

Due to the Kleinunternehmerregelung in Germany, one has to use "Exempt from VAT", which is not a problem. But in addition to that, I either need an exemption reason code (from VATEX I guess), or I need to fill the exemption reason text BT-120.
VATEX does not have a code which works, as I always get the BR-E-10 error. So there should be a custom field which is converted into BT-120.

TLDR: There should be an empty field for example in the e-invoice settings to write a reason for the exempt which is mapped to BT-120 in the e-invoice.

[diedthunder]

Here we have the validation fault for the case above:
[BR-E-10]-A VAT Breakdown (BG-23) with VAT Category code (BT-118) "Exempt from VAT" shall have a VAT exemption reason code (BT-121) or a VAT exemption reason text (BT-120).

For the §19 Kleinunternehmerregelung in Germany one should be able to add "Exempt from VAT due to §19 Kleinunternehmerregelung" in BT-120.

[creamdory]

We are also looking for this field. Can we add this one custom? What should be the technical name needed?

[0xD0M1M0]

@diedthunder & @creamdory Please review the proposed PR (possibly even testing it in your environment) to check if it would solve your problem.