Is one absolved by an expiation for an oath (kaffarat yamin) concerning a vow to donate a large sum of money to charity in order to stop masturbating, or must the vow be fulfilled, knowing that one has the financial ability to fulfill it?

This vow is a "lajaj vow" (a vow made in a fit of anger or stubbornness), and according to the most preponderant opinion, it is not obligatory to fulfill it. Rather, its ruling is that of an oath, so the one who made the vow has the choice between fulfilling it or offering an expiation for an oath. The expiation for an oath has already sufficed you, and your vow is nullified, and you are not obligated to give the mentioned money in charity.