Is it permissible for a mother to grant her eldest son a portion of her inheritance (inherited from the father) in exchange for what he spent on her treatment and expenses, while she continues to be in need of treatment?

If a man spends on his mother with the intention of donating, then it is a gift and he cannot reclaim it. According to many scholars, the mother is not allowed to favor him with anything over his siblings. However, if he spends with the intention of reclaiming the money, then he may reclaim what he spent, and he is permitted to do so in the future.