(Mishnah): One who buys wine from Kusim says (the following declaration before Shabbos in order to be able to drink from it on Shabbos), "Two Log that I will separate shall be Terumah, ten shall be Maaser Rishon and nine Maaser Sheni and then he may redeem it onto coins.

(Gemara) Baraisa: R. Yosi and R. Shimon prohibit it, lest the wine bottle break and he would have retroactively drank Tevel.

R. Yosi's and R. Shimon's reasoning is different. R. Yosi's reason here is that there must be a specific part that becomes Terumah when he declares it. R. Shimon's reason is that he might forget and not actually separate it and drink all of the remainders.

Support (Tosefta): The Maaser Sheni in this thing is redeemed onto a Selah coin in this money pouch, it is redeemed (even though he didn't specify which coin should be used). R. Yosi says that it is not redeemed (because he didn't specify).

R. Yosi agrees that if he said, "on the old Sela'' or "on the old Dinar'', it is redeemed. And the Chachamim agree that if he said, "on the Sela that I will take from my son'' or "on the Dinar that I will take from my son'', that it is not redeemed, as it wasn't in his hands at that time.

(Mishnah in Maseches Gitin): One who put aside fruits to use to tithe for other fruits etc. (money to be able to redeem) Maaser Sheni, (he may tithe with them assuming that they are still intact). But if he then found them to be lost, he must be concerned about any tithing done in the last 24 hours. (These are the words of R. Eliezer.) Isn't this the opinion of R. Yosi and R. Shimon? (If they are concerned that the bottle might break, they are also concerned that the fruits might be lost or burnt).

(R. Zeira): There (Gitin), the potential concern is about what has already happened (that the fruits might have already been lost when you wish to tithe, so R. Yosi and R. Shimon agree that we can assume that they are still currently intact); here, the concern is that the bottle will break (in the future, and one cannot assume that the bottle will always continue to be intact, since bottles eventually break).