Is the amount given by the grandmother considered usury, and must it be returned, especially if her intention was to do good with it, knowing that she had previously said she would return a smaller amount with more?

Lending to a grandmother with the lender's acknowledgment of her statement that she will return an excess amount is a collusion for an increase, and collusion between the lender and the borrower for an increase is forbidden. It is not permissible to take that increase on what was borrowed; rather, only the principal amount should be taken, for Allah Almighty says: "But if you repent, you may have your principal – [thus] you do no wrong, nor are you wronged."