What is the contractual basis for the invoice?​

In our contract you can find the following in the general terms and conditions:
§ Section 5 (3) of the Factoring GTC

In the event of the debtor's insolvency, the purchase price of the receivable shall be reduced by the VAT refund claim which the contractual partner can assert in this case. If the debtor's inability to pay only becomes apparent at a time when the purchase price for the receivable has already been paid, the contractual partner is obliged to repay the VAT portion accordingly.

The refund of VAT has no statutory limitation period and can be conveniently carried out via the advance VAT return.