Software always needs to be updated. Who do they plan to take over that job? What agreement will you have in place for future updates? A better option would b

Software always needs to be updated. Who do they plan to take over that job? What agreement will you have in place for future updates? A better option would be to license the software to them. You could even sell them a lifetime license to the software,and give them the code and allow them to modify it but not re-sell it. This should allow them to do anything they want with or without you, but you still retain ownership rights to the software. Then you charge a yearly maintenance fee to keep the product up to date and patched. Software improvement projects are scoped and charged separately.