Have some function in the code which will scramble the whole thing at a later date. If you actively maintain the code, keep pushing the date. If they pay you

Have some function in the code which will scramble the whole thing at a later date. If you actively maintain the code, keep pushing the date. If they pay you for your work, remove this function. This is a 'security feature' which you could deploy in future situations. I do not believe this particular implementation would be violation of any laws.