What is incumbent upon one who took a goat from his grandmother for Hajj on her behalf, but did not perform Hajj due to old age and incapacitation?

Since you accepted the goat from your grandmother in exchange for performing Hajj on her behalf, and you have not done so, and this lease contract cannot be annulled by the grandmother's death, you must appoint someone to perform Hajj on her behalf from your own money. This is because what transpired between you was a lease contract that obliges you to fulfill it. The scholars have stated that whoever is hired for Hajj must either perform it himself or appoint someone else to do it on his behalf.