Does one have to give out one-fifth of a sum of money as charity or as fulfillment of a vow that its owner did not disclose until after making a profit?

Those involved in a transaction are not required to pay its khums due to a vow made by one of them, as they do not own the entire transaction. As for the one who made the vow, he is only required to pay khums on his share, because he owns it. For a human being cannot vow over something he does not own.