i would imagine he gets paid for what the company buys, because CD stores and whatnot seem to hold on to CD's and wait for them to sell, that way they have more selection than if the store sent the CD's back for a refund.
I think it depends on the chain. The store I work in does a return at the end of every month. For catalogue titles, we send back anything that hasn't sold at least a year. If there are more than one of a given title, we look at the sales for the past six months. If they are good numbers, we keep them both. It they aren't we send back one, or both.
For hit titles it's a bit different. We have to run a sold report and send back overstock or any product that isn't moving. We have to do it in order to keep space on our shelves for newer titles and the things that more people want.
I don't know the specific numbers, but we just had to send back five of the eight copies we had of White Light. It's still selling, but not as strongly as it was for the first two weeks. It seems to have done well by normal standards, but when comparing it to MGB albums the numbers are a lot smaller.
On a completely different note, a lot of the people I talked to at work who bought the cd weren't hard core Matt Good fans. A lot of people who bought it said they didn't like his older stuff much at all.