If I buy a software project from you, I need assurance that I can keep using that project and doing maintenance on it no matter what happens to you or your business. If you die tomorrow, I will need the source code and the ability to modify it as I see fit for the contract to not carry a significant business risk for me. Signing over rights (apart from authorship and other inalienable rights) to your software should be a standard part of any development contract, in case this is not done and you keep ownership, I also expect free maintenance on the code and significantly lower costs(as is common when licensing general use software, such as MS Office, etc). If this piece of software is critical for me, for example part of a contract I made with a client of mine who needs assurance that the software keeps working then it is not possible for me to allow you to keep ownership, sorry.