I'm very new to software contracting; this is my first project. I've just built a point-of-sale software system for a client, and neither of us put a lot of work into the contract. I wrote that the software was "jointly owned" with exclusive license for use given to the client. The client is using it at one store and is very pleased with it. The client is also planning to expand to numerous stores over time, and wants to use it in every store. The client also now wants full ownership of the software, with me as the exclusive developer. I am very hesitant to allow this, and I am seeking previous experience. Should I sell the IP but demand royalties for every site at which it's installed? Should I demand royalties for every sale made using the software? Should I really start talking to a lawyer?

A couple of other details: in terms of risk, it is fair to say that the client is assuming the risk, but the client is now using the software and exclaiming how great it is (and so I assume, how it is improving business). Also, the software is tailored to the client specifically, but could, with a bit of work, be repackaged and resold to other clients. Even if the client owned the IP I would certainly want to make sure that I then did have (significant) royalties on such sales.