If you’re setting it up as a good will gesture, take a look at this topic:
http://community.quickfile.co.uk/t/customer-client-promotional-goodwill-gesture-credits-offered/1064
However, if you have more of a contra-invoicing method (they invoice you, you invoice them and they cancel each other out - therefore no money exchanges hands), take a look at this:
http://community.quickfile.co.uk/t/contra-invoicing-same-customer-and-supplier-netting-off-receivables-and-payables/345/2?u=parker1090