Is one absolved of an oath by paying a certain sum to the one who performed a deed if the rightful recipient refuses to take or return the sum?

The oath you took is an unfulfilled oath. By giving the money to your nephew, you have fulfilled your oath, so there is no expiation due upon you because you swore to give him the money and that happened. This is unless your intention was for him to own the money and benefit from it. In that case, the unfulfilled oath remains if he refuses to take it, and expiation is not obligatory for you except when fulfillment becomes impossible. It is permissible to offer it in advance.