I haven't done it yet, but I imagine you simply point the name to their name servers, and keep it going as long as the payments come in.
No matter what you do, make sure you have a legal agreement in place that outlines all the terms, and accounts for all discrepancies. For example, you don't want to deal with an issue where they missed a payment, you shut down the site, and now their coming at you for their loss of business...
I'm interested in domain rentals myself, hopefully someone more knowledgable can offer some input.