What is the ruling on a father dealing with someone who has entered into an agreement with his extravagant and wasteful son to carry out banking transactions, without the father's knowledge or permission, knowing that the partner is aware of the son's condition and has pledged to bear responsibility for any loss?

Appointing a prodigal (safīh) over money is not permissible, as it exposes the money to loss. I advised the man not to deal with your prodigal son, and this is considered sincere advice (nush) in religion.

Regarding the guarantee of money received from a prodigal: if the principal (muwakkil) was aware of your son's prodigality, then your son is not liable if the money is lost before it is requested. However, if the prodigal intentionally destroys the money in his possession that was given to him by its owner's choice, then he must guarantee it.