Your question is almost entirely about how you and your customer(s) are doing business with each other. We don't know what your understanding is with your customer(s), so nobody here can tell you what is or isn't ok in this situation. It's already clear that you don't trust this customer, so that would indicate it's time to get something in writing. Verbal contracts are difficult to enforce even in countries that recognize them as legal.
However, if the understanding is that you are to be paid for hours spent on the project, then the customer might expect that the product of that labor is already their property (upon payment, anyway) and be rather upset if you intend to charge them extra for it.
The fact this is a sideline for you is pretty irrelevant to the business aspects of the situation.