Would a software developer be sinful if they refused to accept payment for programs they provided to clients who insisted on paying?

There is nothing in gifting software and not taking payment for it that necessitates doubt and inquiry into whether a sin has occurred. It is not a sin, whether the intention from the outset was a gift or a gift of the payment. And if you take something from them thereafter, there is no sin upon you, for it is a reward for the gift.