Is it obligatory for someone who masturbated in his father's or grandfather's house and performed ghusl there, to pay for the water he used for ghusl in his father's house, or does that fall under customary permission?

There is no harm in performing ghusl (definition) (ritual bath) at the homes of parents, whether it is due to masturbation or other reasons, while noting the prohibition of masturbation. Seeking special permission for this or offering compensation for it is considered an obsession and an act of extremism, as custom allows for such acts. Allah has lifted the burden from eating in the homes of relatives due to their leniency, so what about performing ghusl? Allah Almighty says: "There is no blame upon the blind, nor is there any blame upon the lame, nor is there any blame upon the sick, nor upon yourselves for eating from your houses or the houses of your fathers..." [An-Nur:61]. Eating from the homes of relatives is permissible, whether they give explicit or implicit permission, because kinship itself is a form of permission, and people's souls permit it.