Credit system usable in European Union countries

Hello,

we are company based in European Union. Here we have to pay VAT from every received payment including credit which client deposits onto his client account as credit.

We cannot use current credit system in WHMCS because it is not fully legal in EU.

Is it possible to add some EU-friendly credit system?

1) Client adds funds/credit to his account and gets an invoice where the deposited amount is the amount WITH VAT included and only the amount WITHOUT VAT is added to his credit account.

2) When he uses credit to pay any invoice, there should be following scheme:
SUBTOTAL without VAT - CREDIT = TOTAL WITHOUT VAT + VAT counted from the TOTAL.

Thank you!