rava suggested that a smart thing shimon could do is to pay the orphans of reuven with the field and then he'll automatically deserve it back because he made a deal with reuven that if a creditor comes and takes the field he'll compensate. So my question is - this is not a case of a creditor taking it, he's paying them with it for what he owes them. If we're saying here that he gets it back, then he could have also paid reuven with the field when reuven was still alive and shimon would have got it back right away! And that's probably not the law, so then why is that the law by the orphans, what shimon's doing is paying for the field with the field itself and making it like he's a creditor!?
Yaakov Shemesh, staten island, united states
From your question it is evident that you understood that (1) Shimon (the buyer) is returning the very same field that he purchased to the children of Reuven (the seller), and (2) that he wants them to return it to him because they have taken responsibility to refund him if it should be taken away from him (i.e. they have obligated themselves in Acharayus), and it has been taken away from him as payment for its own purchase!
I understood the Gemara very differently. I believe that this is indeed a normal case of a seller refunding a buyer for what the seller's creditor took away from the buyer. And it is clear from the Rishonim that Shimon can pay the orphans with any piece of land that he owns, with the same results. (See Tosfos DH who writes that even if Levi took away from Shimon the field that Shimon bought from Reuven, and not just its value, Shimon can still collect from the orphans by giving them land -- obviously, other land.)
In the Gemara's case, the seller's creditor (let us call him Levi) took either the field that Shimon bought or its value from Shimon. Shimon has thus incurred a loss due to Reuven, the seller. When Shimon gives a field to Reuven's orphans as payment for the original field (which he has since lost to Levi), Shimon can then go back to the orphans and collect it from them as an Acharayus payment. The reason for this (in the Hava Amina) is because the field is retroactively considered to have belonged to their dead father, and to have been inherited thy them - with an Acharayus lean on it to Shimon. Shimon now collects the field from the orphans through that lean.
The Gemara concludes that even if Shimon's payment does not retroactively belong to the dead seller, he can collect his Acharayus payment from the field that he paid the orphans for another reason. That reason is Shi'abuda d'Rebbi Nasan, which tells us that since A owes B, and B owes C, then A owes C. In this case, since Shimon owes Reuven (the price of the field), and Reuven owes money back to Shimon (the Acharayus payment), then Shimon owes himself money. Thus, he pays the orphans with land, and then collects it back from them based on the above logic.