I wondering why the Rambam decided a law like the ''I would have said,” instead of the conclusion of the Gemara in Kidushin pg 28. The answer I had thought is that perhaps the conclusion of the Gemara is going like Rav Joseph that holds that one need to evaluate an object before it can used as having monetary value. [I am referring here to Rav Joseph in Kiddushin page 8. He holds if one marries a woman with silk cloth, it has to be evaluated beforehand. He learns this from a braita that teaches that a Jewish slave can be acquired either by money or what is worth money, but not with vessels or grain. But why not vessels or grain if after all, they also have monetary value? So, Rav Joseph says that the reason vessels or grain are not valid is that their value was not estimated yet.] But then I realized that Rav Joseph is referring only to using something that is worth money, not that which is used in an exchange. It is only on page28 that Rav Judah says that which is used in an exchange has to be evaluated beforehand. But since that is not the law, therefore the Rambam decided to go with the “I might have said (hypothesis that was rejected) at least in the way the Gemara understands the mishna according to Rav Nachman. To make this clear let me bring the Mishna and Gemara. The Mishna says anything that is made money, once one acquires one, the other acquires the other. This, I might have thought, refers to coin. But we know that coins cannot be use in an exchange. R. Judah answered, "It means, what is used in an exchange has to be evaluated." The Gemara then asks why was the “I would have said” a viable possibility in the first place? Answer: It would have meant coin can be used in an exchange, and also fruit can be used in an exchange. But Rav Nachman held fruit cannot in an exchange, so the Gemara concludes the meaning is that if one sells an ox for a hundred, and after that the seller finds out that the buyer has a cow worth that same price, the seller can say, “Give me the cow instead of cash.” And just by saying this, the acquisition is complete. At any rate this Gemara is hard to understand anyway since it says that this last approach where one that sold the ox says keep the hundred you owe me and give me your cow instead, that law is going like R. Yochanan who holds that from the torah money cause a monetary transaction to be valid. I do not understand what just saying keep the money and give me the cow instead has any relevance to the law of the Torah that exchange of coin, not picking up the bought object causes the sale to be valid. In our case here no one ever picked up any coin what so ever. Furthermore, it is possible to say like Tosphot does that the law the Gemara says here is going only to the opinion that coin can be acquired by exchange because Rav Nachman does not say the Mishna means ''all that is evaluated'' (like Rav Yehuda) but leaves it to say ''all that is made''. How could the Rif and Rambam answer this problem? --------------------------------------------------------------------------------------------------------------- I wondering why the Rambam decided a law like the ''I would have said,” instead of the conclusion of the גמרא in קידושין דף כ''ח. The answer I had thought is that perhaps the conclusion of the גמרא is going like רב יוסף that holds that one need to evaluate an object before it can used as having monetary value. [I am referring here to רב יוסף in קידושין דף ח'. He holds if one marries a woman with silk cloth, it has to be evaluated beforehand. He learns this from a ברייתא that teaches that a עבד can be acquired either by כסף or what is שווה כסף, but not with vessels or grain. But why not vessels or grain if after all, they also have monetary value? So, רב יוסף says that the reason vessels or grain are not valid is that their value was not estimated yet.] But then I realized that רב יוסף is referring only to using something that isשווה כסף not that which is used in חליפין. It is only on page כ''ח that רב יהודה says that which is used in an exchange has to be evaluated beforehand. But since that is not the law, therefore the רמב''ם decided to go with the “I might have said (hypothesis that was rejected) at least in the way the גמרא understands the משנה according to רב נחמן. To make this clear let me bring the משנה and גמרא. The משנה says כל הנעשה דמים כיון שזכה זה נתחייב זה בחליפיו. This, I might have thought, refers to coin. But we know that coins cannot be use in an exchange. רב יהודה answered, "It means, what is used in an exchange has to be evaluated." The גמרא then asks why was the “I would have said” a viable possibility in the first place? Answer: It would have meant coin can be used in an exchange, and also fruit can be used in an exchange. But רב נחמן held fruit cannot in an exchange, so the גמרא concludes the meaning is that if one sells an ox for a hundred, and after that the seller finds out that the buyer has a cow worth that same price, the seller can say, “Give me the cow instead of cash.” And just by saying this, the acquisition is complete. At any rate this גמרא is hard to understand anyway since it says that this last approach where one that sold the ox says keep the hundred you owe me and give me your cow instead, that law is going like ר' יוחנן who holds that from the תורה, money cause a monetary transaction to be valid. I do not understand what just saying "Keep the money, and give me the cow instead," has any relevance to the law of the תורה that exchange of coin, not picking up the bought object causes the sale to be valid. In our case here no one ever picked up any coin what so ever.------Furthermore, it is possible to say like תוספות does that the law the גמרא says here is going only to the opinion that coin can be acquired by exchange because רב נחמן does not say the משנה means ''all that is evaluated נישום '' (like רב יהודה) but leaves it to say ''all that is נעשה דמים באחר''. How could the רי''ף and ר''ם answer this problem?