Why is it that in the case involving Shimon paying off the ysomim's father's baal chov, he cannot recollect his money from the baal chov? He does not have a mitzva to pay back the ysomim's father's debts?!
S First, usa
(1) The point here is that Shimon made a mistake when he paid off the yesomim's father's (Reuven's) baal chov. The reason he paid him is because he thought that since he had never paid Reuven for the field, but rather remained in debt to Reuven, he would therefore pay up for the field, and at the same time remove Reuven's baal chov's complaint that Reuven had never paid him back. Shimon's intention was not to fulfil a mitzvah by paying up Reuven's debt to the latter's baal chov, but rather to achieve 2 things with one shot - to pay up his own debt on the field and to appease Reuven's baal chov.
(2) However Shimon miscalculated. Because Shimon owed Reuven money before he died, this means that after he died, Shimon now owed the orphans money. However even after Shimon paid the orphans they had no obligation to pay up their father's debt to his baal chov, because they had inherited no land from their father, and the halacha is that there is no obligation for the orphans to use their "metaltelin" (mobile items) in order to pay up their father's debt. Therefore the orphans now argue to Shimon that the money that you paid to our father's baal chov has nothing to do with us, because we have no connection with our father's baal chov since our father left us no land. Consequently, the orphans say to Shimon that you still have to pay us the money you owed our father for the field, and the money that you gave to our father's baal chov was effectively merely a present for him. If you come to us with a complaint that our father promised you that if anyone should claim his debt back from the field he sold to you, he will reimburse you, we will deflect you by arguing that we have no responsibility for the field that our father sold you, because he left us no land.
(3) It could be that theoretically Shimon could try and argue in court what you suggest - that since he gave the money to Reuven's baal chov by mistake (because he has no mitzvah to repay Reuven's debt) this is considered a "mekach to'oos" (a mistaken transaction) and he wants it back from the baal chov. However this is going to be a complicated proceedure, because the baal chov is clearly going to contest it. Therefore Rovo suggested a more effective tactic. When the orphans claim their money back from Shimon he should pay them back with land instead of cash. This means that the orphans do now possess land that they got from their father and therefore Shimon can now come back to the orphans and claim back the field from them because their father promised that if anyone would come with claims on that field, he would protect Shimon. Now that the orphans possess land from their father, they also have to fulfil that promise, and consequently they must return the field to Shimon.
(4) In short, possibly one could argue that Shimon could get the money back from Reuven because he had no mitzvah to give it to him, but Reuven is going to make things difficult for Shimon if he tries to do this so Rovo suggested a better idea for Shimon if he is astute :- give land to the orphans and this will automatically commit them for their father's responsbilities.