Dear Rich response.
As a General Rule Dept. Signing an assignment for the “promise” of future payments always involves a risk. We used to have a client whose golden rules were, "Get as much as you can before signing the deal," and "Don't give any money back." One reason to get the money upfront is that if the assignee (the person who owes you the money) fails to pay you, you may be trapped in dispute resolution, fighting to get your copyright back. If the assignee insists on a series of payments, there are several solutions, such as establishing an escrow account, transferring partial assignments per payment, or setting up a license agreement that allows for ownership transfer after the final payment. This is a situation where you should consider contacting an attorney (and we hope your assignment agreement has an attorney fees provision).