I would add, that in order for that to be true, there likely needs to be an agreement in place. If the money was received unexpectedly, say, it was not stated in an agreement or contract for future services, then the recipient could have returned the payment, since in that case it could be they do not have an explicit "right" to the payment, and a performance was not completed to earn the right, in absence of a contract that offers the right to it. Even then, timing of the payment should be listed in a payments schedule along with performance obligations( if any), so that there is a more determinant manner to whether there is a right or not. This would be important to protect both parties.
In this case, it seems like there likely was and someone just wants to defer the taxation for personal gain reasons alone, and making up a false pretense that it was for future performance, without mentioning contractual duty.
