1) I have found that the Ketzos ha'Choshen cites Tosfos 123b, DH Hacha, in three different places: in 204:3, 207:9, and 278:15, so I am not sure which place you are referring to.

2) However, Tosfos himself explains how Makirei Kehunah can work without a Kinyan. This is because the present that one gives to a Kohen is considered to be only a small present and one is not allowed to retract from a small present, even though it was only promised verbally.

3) The source for this is Gemara in Bava Metzia 49a, where Rav Papa said that Rebbi Yochanan agrees that even if someone merely said that he is going to give a small gift, he may not retract, because the recipient relies on this promise (if he promised a large gift the recipient does not believe that the donor will really give it without a Kinyan, so the donor is not considered unreliable if he retracts).

4) Tosfos adds that even though one is not allowed to retract on the small gift, nevertheless if he does decide to retract this is effective. However, the prohibition l'Chatchilah to retract means that as long as he does not retract, the recipient is considered as "Muchzak."

5) When Tosfos says that he is Muchzak this does not mean that there has been a Kinyan, but rather what we are referring to is the Din that the firstborn receives a double portion only from what the father was Muchzak in before he died, but not from what is merely Ra'uy to come. If the animal had been slaughtered when the Kohen father was still alive, since the owner of the animal knows the Kohen and always gives his presents to him, this means that the Kohen is Muchzak in these presents since the owner of the animal is not allowed to give the presents to anyone other than the good friend, the Kohen. The Kohen, or his children, cannot force the friend in Beis Din to give them the presents but the fact that the Yisrael is not allowed l'Chatchilah to give it to any other Kohen means that the Kohen is Muchzak since he can assume that the present will come to him.

6) After further thought, I must confess that at first I did not appreciate this question properly. The point is that Tosfos does not write at all that there is no Kinyan, but the Ketzos ha'Choshen (204:3) is in doubt about whether Makirei Kehunah require a Kinyan and then cites our Tosfos to show that it does not require a Kinyan. We see that the Ketzos learns that according to Tosfos, Makirei Kehunah works without a Kinyan. The Ketzos (207:9) also cites Tosfos and immediately writes that there is no Kinyan with Makirei Kehunah. So we see that in two places the Ketzos writes that according to Tosfos, Makirei Kehunah does not work with a Kinyan, even though Tosfos himself does not write explicitly that there is no Kinyan.

This led you to think that the Ketzos has a different Girsa in Tosfos. I do not think that the Ketzos has a different Girsa from ours. If one looks at the three places where the Ketzos cites Tosfos (as quoted above), one notices that the Ketzos never cites Tosfos himself as saying that there is no Kinyan, but rather when he explains Tosfos he writes that Makirei Kehunah works without a Kinyan.

7) I therefore remain with the basic Pshat that I gave above (#4) to explain the Ketzos: The prohibition l'Chatchilah to retract means that as long as he does not retract, the recipient is consdered as 'Muchzak.' The fact that he is Muchzak makes a Nafka Minah concerning the firstborn receiving double, as I wrote above. The Ketzos writes clearly (in 278:15), "It is explained explcitly in the words of Tosfos (123b) that Makirei Kehunah is only because of the prohibition of retracting." The Nesivos ha'Mishpat (278:11) also writes that according to the Ketzos, Makirei Kehunah is Muchzak since it is forbidden to retract.

Dovid Bloom