What is the ruling of the Sharia on one who withholds financial aid that he used to provide, preventing it as a punishment, while remaining capable of giving, and is this considered a loss in trade with Allah?

It appears your uncle promised you a monthly allowance. It is most probable that a promise is not obligatory to fulfill, but rather recommended and desirable. However, fulfilling it may become necessary if its non-fulfillment would lead the promised person into hardship. What your uncle used to do was an act of kindness and benevolence that deserves gratitude, and his cessation of it does not erase his previous kindness. Try to persuade him and appease him so he may resume his connection. We advise you to be dutiful and kind to your mother, and we warn you against excessive arguing with her or raising your voice at her, as that is an act of disobedience. As for anyone who considers himself the giver and the preventer, if he believes he does so without Allah's will, then that undermines his faith, for the true Giver and Preventer is Allah. It is not permissible to think ill of your uncle merely because he stopped providing what he used to give you.