At the recent pidyon ha'ben made by my nephew for his bechor, my BIL RYGB started discussing with me why you have to make a bracha on the mitzvah of pidyon ha'ben. Rashba writes in a teshuva that we don't say a bracha on mitzvos like tzedakah where the fulfillment depends on other people -- since the poor person can turn down your gift, you risk it being a bracha l'vatala. Since pidyon ha'ben depends on the kohen taking the money (you are not yotzei just by doing a hafrasha -- you have to actually give the kohen the money), it too should be classified as a mitzvah that is taluy b'yad acheirim and not require a bracha.
I asked my son what he thought, and he immediately remembered the Ketzos (243:4) has a chidush that nesina ba'al korcho works when it comes to paying off a debt and matnos kehunah. You don't need the kohen to cooperate -- you can force him to take the money.
I thought you could distinguish between tzedakah and pidyon ha'ben. When it comes to tzedakah, if the poor person does not want/need the money, there is no chiyuv. With respect to pidyon ha'ben, the chiyuv exists whether the kohen cooperates or not. His refusal to accept payment deflects the kiyum mitzvah, not the chiyuv.
RYGB wanted to say a bigger chidush and suggest that the kohen has a mitzvah to accept the payment. I did not believe this (see Pesachim 121 where the gemara says with respect to pidyon that the father has the mitzvah; the kohen gets hana'ah) but I found that R' Noson Gestetner in his sefer on chumash on last week's parsha compares the role of the kohen to that of a woman in the process of kidushin -- there is a hechsher mitzvah or a kiyum of some sort, even if there is not a full chiyuv. He writes that m'sevara this has to be the case. How could a yisrael be commanded to do pidyon if every kohen in the world could theoretically turn down his money? How would he do the mitzvah? It must be that the kohen has an obligation of some sort to participate.
Let's test the theories by applying them to another mitzvah: mishloach manos.
You can't force someone to accept mishloach manos, so it makes sense according to the sevara of the Ketzos that there is no bracha.
According to my approach, it's a little trickier. I have a chiyuv of mishloach manos irrespective of someone else's need for my gift, so isn't it like pidyon ha'ben?
The Manos haLevi writes that the purpose of mishloach manos is to engender friendship between people. That seems to be the logic behind the view of the Rama that the recipient can be mochel the gift -- it's the thought that counts, not the actual present. If so, perhaps no bracha is recited because the essence of the mitzvah -- friendship and collegiality -- is devarim she'b'lev, and no bracha is recited on devarim she'b'lev.
According to RYGB's sevara, it's also a little trickier. If you hold like the Rama, you could simply say that there is no mitzvah on the recipient to accept anything since he can be mochel. But I wonder how you would resolve this question if you don't hold like the Rama. Using R' Noson Gestetner's logic, since I have to give mishloach manos to somebody, doesn't there have to be some mitzvah on the recipient to accept it, or theoretically at least I would never be able to fulfill my mitzvah? (I did not discuss this last point with RYGB.)