When you enter a purchase invoice with a positive amount, you record that you owe the vendor money.

When you enter a purchase invoice with a negative amount, you record that the vendor owes you money.

This will then show on the Vendor Aged Detail report as a negative amount.  It can then be used to record against future invoices in the Payments module so that future invoices are shown as paid and the amount of the credit is reduced.

Thanks for your reply. This is pretty much the way I was going, but I was wondering if there was a more proper way of doing it. The only way I could work it was to post the neg. invoice and each subsequent positive invoice, but not post any payment until the balance was reduced to zero. 

You can post payments so the subsequent invoices are marked as paid.  Or you can leave them all outstanding as you say, until you have a positive balance owing.

If the number of invoices is greater than 24, you may have problems printing them all on the one cheque.