The discussion of whether a kohen can forgive the debt of pidyon haben may be dependent on another point. How do we view the forgiving (mechilah) of a debt? One option is that when one says that the debtor doesn’t have to pay the debt, the debt is off. The other option is that when one says, “you don’t have to pay me,” we view it as if he has already received the money.
If forgiving a debt is only a means of removing the debt, one can argue that it should not work by pidyon haben. This is because the money is not owed to only one person, but rather to the entire sheivet of kohanim; one kohen should not be able to remove a debt that is owed to the entire sheivet. However, if forgiving a debt is considered as if he has received the money, one kohen can forgive the money. This is because it is no different than if he had actually received the money – in which case the debt is off.