What is the ruling on money that was donated for building a mosque, then the agent spent it on himself, and can its intention be changed to become charity for the agent, or does it remain a trust that he must return by building the mosque?

Your father should have fulfilled the trust and built the mosque. However, if you forgive him and do not demand the money, then there is nothing upon him. You are not obligated to build the mosque merely by intention and by setting aside the money, unless you made the money an endowment (waqf), or it was placed on a piece of land with the intention of building a mosque upon it; because it is not permissible to revoke an endowment after it has been taken into possession (hayaza).

We advise you to forgive your father out of filial piety, and to strive to build a mosque if you are able, and encourage him to contribute. Changing one's intention regarding charity is permissible if the money has not reached the poor person. If your father took the money and spent it, then it is a debt upon him, and you may give him this debt as charity if he is poor and cannot repay it.