I wouldn't sell it to them outright. Potentially they have done due diligence on selling your software and have decided to buy the software from you and sell it themselves. If you are willing to take the risk, push them to license the software from you as a framework. Take a look at the models that ATG (now oracle), and sword-ciboodle crm have. They are established enterprise models where the framework is licensed to a company and the company is free to make changes of their own (but not resell). When you put in future updates you can sell them new licenses for the later versions.