[FIX] account payment difference sign

When a payment is registered on an invoice, the payment difference must
be considered differently if we receive the payment or pay a bill. This
is necessary at the creation of the 'Counterpart' entry. For example,
an invoice of 100 is considered as paid with a payment of 90. The
difference of 10 should be booked as a debit in case of a customer
invoice, but it should be booked as a credit in case of a vendor bill.

opw-675641