Baba Kama 36b - Pledging Money that is not in one's possesion

The gemara says that when one is owed money as compensation for embarrassment, and they declared that the money should be given to tzedaka, their word is binding. Why? Because the beis din serves that the "hand" of the poor, and were already zocheh it for the poor. Tosafos asks, why not say that even if beis din would not be zocheh for them, it would anyway have to be given to the aniyim because it is as if he made a vow to give tzedaka? Tosafos answers that since when he said that it should be given to aniyim, it wasn't yet in his hands, it is not binding as a neder to tzedaka. But if he would say "when i get the money, i will give it to tzedaka", Tosaofos says that he would be obligated to fulfill his neder. Tosafos then quotes Rabbeinu Chananel who seems to assume that even if he would say "it should be to tzedaka", it would be a binding neder, but he would still be able to borrow the money for the time being. But, Rav Yosef says that he was already zo'cheh for the aniyim through ma'amad shelashtan.

Both answers of Tosafos are paskened in Shulchan Aruch (Y.D. 258:8). If one is owed a debt and sya that it should be for tzedaka, it is not a binding neder, but if he says that when he collects it, he will give it to tzedaka, then it is a binding neder. BUT if the money is in the hand of the ga'bai tzedaka, or if he says it in the presence of the gabai and the ba'al chov where ma'amad shelashtan goes into effect, the gabai acquires it for the aniyim.

The Shach (18) questions why Tosafos couldn't answer simply - If it were binding as a neder, one could be matir neder and uproot the chiyuv, but since it is binding as zechiya, he can't uproot the neder? The Shach proves from here that anytime one pledges money that is in someone else's hands, he cannot be matir his neder since they will automatically be zo'cheh for the aniyim. Hataras Nedarim will only work on money that is in the hands of the pledger, but not on money that is in the hands of someone else.

Can someone retract on a pledge to a tzedaka, based on the pe'sach of "if i would have known that the market would crash, i would never have pledged so much"? Based on the shach, if the money pledged was in his own hands (or in the hands of someone who cannot be zo'cheh for aniyim i.e. goyim), he can retract. But, if it is in the hands of another jew, who can be zo'cheh for aniyim, he cannot retract.