What is the ruling on swearing by Allah to prevent the fiancé from coming to the office, then him coming and paying a sum of money as expiation for that?

Since you swore that someone would not do something, and then he did it, you have broken your oath and must offer an expiation for the oath (kaffarat yamin). It is permissible for someone else to pay it on your behalf with your permission. It is commendable to help a Muslim fulfill his oath, unless doing so causes hardship. Your fiancé, before the marriage contract, is considered a stranger (ajnabi). It is not permissible to be alone with him, uncover in front of him, or speak with him without necessity.