FITV in SAP Portal - tax code depending on country/region and expense type

Hi experts,

I have a quite difficult topic here. My client in Portugal has a specific requirement regarding tax code to be applied for each expense type.

For domestic trips in Portugal you can have Portugal (Continental), Azores and Madeira (islands) and in Portugal you have different applied tax codes from the ones that are applied in Islands (Azores and Madeira). I mean for example for car rental you have a 23% tax code and in Azores and Madeira you have 19%.

In ESS Portal they don't want to employees be able to select the corresponding tax code so we will disable tax code field (see screenshot attached), so the corresponding tax code should be determined automatically by the system but the system only determines the corresponding tax code by expense type.

http://screencast.com/t/zuCFiG16kgsF

We were thinking if we can have a custom table behind with country/region, expense type and tax code to apply and then using a BADI to change the corresponding tax code to be applied. I've searched the documentation and I found BADI definition AFTER_TRIP_SETTLEMNT but I am not sure if it would work.

Do you have any hints to solve this topic?

Thanks in advance.

Cheers,

AS