Is it obligatory to help one's father negotiate with tenants regarding the costs of repairs, knowing that the father might exaggerate the costs and that the repairs are not necessary?

It is not permissible for a father to take the costs of repairing or renovating a property from tenants without their consent, because the repair of the property is the responsibility of its owner. This falls under unjustly consuming people's wealth.

The questioner must advise his father and explain this to him, and seek assistance from someone who holds influence with the father.

If the father does not respond, then it is not permissible for the questioner to assist him in this, due to the Almighty's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression," and due to the Prophet's (peace be upon him) saying: "There is no obedience to a creature in disobedience to the Creator."