What is the ruling on one who decreases or increases the agreed-upon amount for Hajj on behalf of the deceased, and is the doer of good rewarded for his benevolence in this situation?

Muslims are bound by their conditions. If the payer and receiver stipulate the return of any excess and the completion of any deficit, then each of them must fulfill their commitment. If there is no such condition, the receiver takes the excess and makes up the deficit. He will be rewarded if he takes the money with a good intention and performs the duty.