"So too something worth money needs to have a fixed, known value". As for what we say later in Kiddushin 12a: "A man betrothed [a wife] with bundles of tow cotton. Rav Shimi bar Hiyya said and examined it—if it was worth a perutah, yes [the betrothal was effective]; if it was not, no". It seems like [Rav Shimi bar Hiyya] does not agree with Rav Yosef [since he allows the evaluation after the fact, while Rav Yosef requires the evaluation beforehand].

"What is it? Symbolic exchange (halifin)." A Hebrew slave can't be acquired through halifin and can't acquire himself through halifin, since the reason he can acquire with money is derived from this verse of "kesef miknato" later (on Kiddushin 14b). But a Canaanite slave can be acquired with halifin, as it teaches in the chapter Ha-Sholeah (Gittin 39b) regarding [the slaveowner] who took his hat and threw it at her and said to her: "Acquire this and acquire yourself", and it is implied there that his act did nothing because it was an item [the hat] belonging to the one who acquired [the slaveowner]. However, if it were an item of the acquirer [the slave], then she would have acquired [herself] through the laws of halifin, not the laws of money. This is also how Rabbenu Hananel explains it there.

"From where do I know this? As it is taught in a baraita: This calf..." He is bringing evidence according to the first version [of the sugya, where Rav Yosef agrees that one does not need an evaluation if he said "It is worth the minimum amount", which there is a perutah but here is 5 selaim. We know that he doesn't need an evaluation because, according to this baraita, all he has to do is to announce that the calf is worth 5 selaim and that is enough, because, like the silk which is obviously worth a perutah, the calf is clearly worth more than 5 selaim. The previous "From where do I know this?" of Rav Yosef at the top of the daf was within the world of "They also disagree if he said it was worth the minimum amount", so Tosafot is clarifying for us that we have returned to the earlier version of the mahloket].(See the Meharsha for his comments on the difficulty of this Tosafot and to resolve it in another way).

"Rav Kahana took a turban [as payment] for redemption of a firstborn son". This implies that Rav Kahana was a kohen. Problem: At the end of Elu Ovrin (Pesahim 49a) it imples that he was not a kohen, since he said, "If I hadn't married a kohenet, I would have not been exiled"! Solution: There were 2 Rav Kahanas. Alternative solution: It was because of his wife that he could purchase [i.e. redeem the firstborn son], as we find in chapter Ha-Zaro'a (Hullin 132a) "Rav Kahana eats [terumah] because of his wife".

"But for general people—no". Problem: Why doesn't this work!? Is it worse than a gift with the understanding that you need to return it, that we say earlier (in Kiddushin 6b) that it is an effective gift!? Solution: Yes indeed it is worse, for there (in 6b) it was a significant thing that was the gift [a maneh], even though it was given with the understanding that it should be returned. But here, regarding redemption, the turban wasn't worth 5 selaim and so when he gave it to him he did not fulfil his obligation of giving [whereas had he given him a maneh, 25 selaim, then he would have fulfilled the technical giving requirement even though he had to return it afterwards].

"Since it wouldn't be kiddushin with a nonspecific maneh, we need [to learn the case with] a specific maneh!?" Problem: Perhaps this is what it is teaching us: A specific maneh, even though she didn't change her mind [like in the reisha], it wouldn't be kiddushin because it turned out to be lacking [less than the promised maneh, and therefore the seifa is in fact necessary because otherwise we wouldn't know about the case where she didn't change her mind]! Solution: You can learn this from the reisha since, [there,] with a nonspecific maneh, she is able to change her mind and we don't say that he should complete [the maneh]—therefore, the kiddushin does not take effect at all until he completes [the maneh, in other words the main point of the reisha is that he hasn't paid yet, not that she is able to change her mind]. So too if he said to her "With this specific maneh" and it turned out to be lacking, the kiddushin did not take effect and even the completion was not effective, for she was not expecting him to pay her anything other than the maneh that he showed to her and it was lacking. [Therefore we can derive the law of the seifa on a specific maneh from the law of the reisha on a nonspecific maneh and the seifa is indeed unnecessary, the Gemara's resolution up to here works (though will be subsequently rejected).]