Hi Ellen,
If there is no withholding tax involved in payment country, go to Vendor/Customer tx FK02/FD02 and:
General Data -> Control - Customer set up your customer
Company Code Data -> Clearing with customer Checked
with this configuration the system will consider Customer/Vendor as one.
If you don't have the fields in your account group, add them in OBD3.
If there is withholding tax, you need a business process posting to a G/L Account
Kind Regards