the penalty of kofer

The value of a person can be pledged to the Mikdash (e.g. dmei ploni alay) and calculated based on the price a person would fetch on the slave market. The Mishna (Archin 20a) tells us that if the person who is the object of the pledge dies, the pledge cannot be redeemed because a person's worth as a servant cannot be calculated after death.

This halacha seem to contradict another din. If a shor mu'ad kills a person the owner must pay a penalty, kofer. There is a machlokes whether kofer is calculated based on the value of the mazik, the owner of the ox, or based on the value of the nizak, the person killed. If we can assess the value of the nizak after death for purposes of kofer, why can we not assess the value of a person for purposes of redeeming a pledge to the Mikdash?

Tosfos answers that in the case of kofer there is a special gezeiras hakasuv that allows and demands that an assessment of value be made. Rashi suggests a logical distinction between the cases. In the case of an ox which gores the owner becomes liable the instant the damage is done; the assessment is just a means of determining how much to pay. In the case of a pledge, the assessment itself creates the obligation to pay; it is not just a means of determining how much cost is involved.

Why does Tosfos resort to saying this is a gezeiras hakasuv instead of accepting Rashi’s logical distinction? Perhaps Tosfos holds that the obligation of kofer is not a natural result of the crime, but must exists only after the court assess the damage (the value of the nizak) and imposes its penalty.