The Birchas Shmuel (Kid 5) explains Tos question like this: the father has a debt to the sheivet kohanim of $5. An individual kohen who accepts a matanah al menas l’hachzir is in effect forfeiting money owed to the entire sheiveit. If the Torah allows an individual kohen to dictate the terms of the debt to the sheivet and to even surrender money, why can an individual kohen not decide that a token item is worth $5 to him and accept that in fulfillment of the debt? Once we allow individual choice to dictate the terms, what are the boundaries?
The answer is that there are two dinim in pidyon: 1) a debt; 2) an obligation to do a ma’aseh nesina. A kohen can define the terms of the debt, but only once the father fulfills the Torah mandated obligation to do a ma’aseh nesina.
I was wondering if this helps use understand the machlokes Avnei Miluim (29:13) and Minchas Chinuch (392) whether you can fulfill pidyon haben by giving a mashkon to a kohen. The Av"M writes that even though a mashkon cannot be used to effect kiddushin or make kinyanim, it can work for a pidyon haben. (I think you have to assume that giving a mashkon for $5 is worth $5, otherwise there is nothing to talk about -- Tos. sevara would preclude mashkon from working just as it precludes the token item.) The M"Ch argues. A mashkon is nothing more than a shibud, a reinforced IOU if you will. A shibud for payment already exists by virtue of the mitzvah -- giving a mashkon adds nothing to father's already existing obligation.
In the case of a token gift, the kohen wants to forgive the balance of the debt without the father doing a full ma'aseh nesina. The case of mashon sounds like the flip side. The debt of $5 is unchanged (shibud before giving the mashkon, shibud afterwards, as the M"Ch argues), but in this case the father has done a ma'aseh nesina by giving a mashkon.