Is it permissible to take from money designated for charity, given with the intention of returning it to the donor, and does that diminish the reward of the charity?

It is permissible to borrow from one's mother to give money in charity. The reward for the one giving charity is complete, and the mother receives the reward for lending. Her action is good so that the charity is not delayed from reaching the needy. The money is a debt owed by the borrower. If the mother permits a delay in its repayment, there is no harm and the reward is not diminished. If she absolves her of the debt, then the reward for the charity goes to the one who gave it, and the mother receives the reward for absolving the debt.