I can't remember the whole deal but a buddy of mine had to give back a deposit plus interest that should have been charged on a car. The deal was that once an agreement is made to accept an installment of the sale price that interest from the seller should be charged. In the event the seller is going to return the funds, the interest should be included. However, I think he is assuming that the interest in this case would be like a savings. You could actually due the math and write it up that the money you have is the total of the hold plus interest on the installment payment.
Best advice, consult local legal council to get advice on the deal.