As someone else mentioned, if their expertise is not in selling software, you shouldn't even considering entering a partnership with them. There is absolutely nothing in it for you. Give them two options: 1. Do the work as requested from scratch. 2. License or purchase the software from you. Which of these you offer should be your choice, not theirs. If you think this software has wide applicability and think you can "sell" it to other customers, you may wish to go the licensing route. Otherwise, if you don't care to retain ownership of the source code, you can sell it to them for a significantly larger amount (at least make sure you've recovered your development costs).