What is the ruling regarding a son who used to give his mother a monthly allowance while falling short in his duties towards his wife, then cut off this allowance from his mother, even though her husband is still alive?

This man should give his mother from his wealth, and he will receive a great reward for it. This falls under the category of showing kindness to parents, which is commanded by Islamic law. This is conditional on it not affecting his obligatory spending on his wife, for the wife's right takes precedence if he cannot combine both, due to the Prophet, peace and blessings be upon him, saying: "If Allah gives one of you goodness, let him begin with himself and his family." And due to the author of Kashshaf al-Qina' saying: "Whoever does not have a surplus beyond what suffices all those whose maintenance is obligatory upon him should begin by spending on himself. If he has a surplus for one or more, he should begin with his wife, because her maintenance is obligatory as a form of exchange, so it takes precedence over sympathy."