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The braisa had stated: If it is an issar less, it is forbidden.
Abaye explains this to mean that the coins cannot be used at its face value if the sela became lacking by an issar more than its price fraud limit.
Rava challenges this interpretation, for if it is even a little more than the limit, it cannot be used at its face value!?
Rather, Rava understands it to mean that if the sela became deficient by an issar to a dinar, it cannot be used any longer at its face value. This would be an anonymous ruling which follows Rabbi Meir’s viewpoint.
Tosfos understands in Rashi that one would be permitted to defraud his fellow if it is less than a sixth, even in a case where he does not intend to return the overcharge. This is because the defrauded party, because it is insignificant, is immediately mochel the “cheater.” It would be permitted to charge exactly a sixth more than its price only if he intends to return the overchatge within the time it would take the other party to show the purchase to a merchant.
The Ritva writes that it is forbidden to defraud your fellow in cases where it is precisely a sixth. This is because people are generally particular regarding these things.
The Ramban maintains that it is forbidden even if it is less than a sixth, for one is not allowed to defraud his fellow whatsoever. If it was less than a sixth, the sages ruled that he is exempt from paying it back. That does not make it permitted.
The Chinuch holds that there is no Biblical prohibition when one defrauds his fellow less than a sixth.