Is it permissible, in Islamic law, to waive all due financial rights and the son's full maintenance in exchange for the ex-husband signing a document by which he relinquishes his right to the son's custody in the future?

The child's maintenance is the father's responsibility without dispute. Custody is the mother's right unless she remarries or has an impediment. The preponderant opinion among us is that when a boy reaches seven years of age, he is given the choice between his parents, unless the father's unrighteousness is proven, in which case he has no right to custody. Regarding the father's relinquishment of custody in exchange for the mother bearing the maintenance, this depends on the jurisprudential disagreement as to whether custody is a right of the custodian or of the child. If it is said to be a right of the custodian, then relinquishing it is permissible. In any case, the Sharia judge is the one who decides matters of custody and maintenance in cases of dispute.