This summary of the beginning of the Pidyon Haben (redeeming the first born son) is on wikipedia
In the traditional ceremony, the father brings the child to the Kohen and recites a formula, or responds to ritual questions, indicating that this is the Israelite mother's firstborn son and he has come to redeem him as commanded in the Torah. The Kohen asks the father which he would rather have, the child or the five silver shekels which he must pay.
When I went to my friend's sons pidyon, the father hesitated when the Kohen asked him the question. After the Kohen proded him for an answer, my friend said, "Wait a minute. I'm thinking it over!"
At the end, he redeemed his son, of course. My friend has a strange sense of humor...
But I'm curious... If he were serious and said to the Kohen, I'm giving my son to you. What happens? Does the Kohen now bear the responsibility of raising the boy? If it is assumed that every father will redeem his son, as the text of the ceremony indicates, then why ask the question in the first place if the Kohen knows the answer?