Hi @graemepirie
If they are both your supplier and client, the best approach here may be contra-invoicing - they invoice you, you invoice them and pay the net value. This is a common approach and has come up several times before on the forum.
Take a look at some of these topics, and hopefully they'll be able to help:
http://community.quickfile.co.uk/search?q=contra-invoicing
The other option is that you add credit to their account, but you would need to account for this coming from somewhere (e.g. a bank account or credit note). This can be done under the client control panel:
But this would be seen as income coming in to you. The contra-invoicing is probably your best option. However, there are many users on these forums, no doubt someone has the same situation and may even have a better alternative than what I do 