Positive and negative amounts on one invoice

Hello,

I have received an invoice from a supplier. It states one amount that I am owed by him (i.e. a negative amount) with 0% VAT. In addition, the invoice shows various amounts to be paid with 21% VAT. The total amount to be paid is small (but less than the total VAT that can be offset). Dexter does not accept positive and negative numbers on one line, and there is no option to split an invoice over two or more lines. How do I record this?

Thank you for a solution that avoids the tax authorities asking awkward questions.