Allow negative amount in invoice

How is it possible to create an incomming invoice (expenses/bills) with a negative amount?

A supplier made a mistake in one invoice and to correct that payment he send a second invoice with a negative amount. But it's not possible to enter negative values into the amount field.

Since it is kind of incomes/revenues I tried that as well but there I am only able to select a customer, not a vendor.

Hello? Is anybody here in this forum?