This question already has an answer here:
During the Pidyon Haben ceremony the Kohen asks the father מה בעית טפי ליתן לי בנך בכורך... או לפדותו בעד חמש סלעים. A free translation of this is -what would you rather do, give me your first born or redeem him for 5 coins?-
If the father chooses to keep the coins over his son, is he mevatel the Mitzvah of redeeming his Bechor? Meaning, are there 2 parts to this Mitzvah - a)keeping the coins or b)redeeming his son and as long as he picks one of them he is Mekayam his Mitzvah, or is the Mitzvah to actually redeem him from the Kohen and if this was not done he loses the Mitzvah. If we say he lost the Mitzvah can he rectify it by redeeming his child at a later point?