Commentary for Bava Metzia 11:9

Money can be returned, [as opposed to an oath, which once uttered can never be retracted]. The Gemara says that money can be returned. In the context of our discussion there are two possible ways of interpreting this satement;
a) Shimon might seize the money or garment with the intention of investigating whether Ruvain truly owes him or not. When Shimon realizes that there is no way that he can reach a decision he will return the money, because he truly does not want to keep any money he is not sure belongs to him.
b) Shimon might seize the money with the intent of investigating whether it is truly his or not, but will not return it to Ruvain unless he is certain that it belongs to Ruvain. If Shimon will remain in doubt, he will keep the money in question.
We must keep in mind that Tosfos maintains that Abaye disagrees with R’ Yochanan. He maintains that if Shimon is suspected of stealing, he is also suspected of swearing falsely. We may assume that whatever level of violation Shimon would perpetrate when stealing money, he would also perpetrate that same level of violation when swearing falsely, i.e. if he would definitely steal, he would definitely swear falsely, if he would steal when in doubt, he would also swear falsely when in doubt.
If possibility b) is true, Shimon would keep the money even if there was a legitimate doubt whether he was allowed to do so or not. So too, when faced with the possible violation of swearing falsely Shimon would do the same. In this case, since Shimon is ready to steal when in doubt, he would also swear falsely when in doubt. Abaye could not suggest that we would allow a person to take an oath, when we know that he would swear falsely even if he was in doubt.
In possibility a) we said that Shimon would not keep the money if he was in doubt whether it belonged to him. In a parallel situation regarding an oath, Shimon would certainly not swear falsely if there was any doubt whether the money belonged to him. Therefore, Tosfos says we must explain the Gemara as discussing possibility a). Shimon may be seizing the garment with the intention of searching for information about the questionable loan in order to remind himself of its status, and if eventually he will not recall that Ruvain definitely owes him the debt, he will return it. He will not keep it if in doubt whether it truly belongs to him. So too, he will not swear falsely if there was any doubt in his mind whether he was truly entitled to keep it. Shimon can seize the money with the intention of returning if he does not recall that Ruvain owes him the money. Returning the money will correct the injustice of taking it illegally, but he cannot swear that the money is his even if he intends to return if he cannot determine that Ruvain owes him because the oath is not retractable.
However, we should not explain that the Gemara means possibility b), that if [Shimon] reminds himself that [Ruvain] is not liable, he will return it, but if he does not remind himself that the opponent is not liable he will keep it even though he knows that he will never remind himself, for if so, since this person treats the prohibition against swearing falsely exactly the same as the prohibition against stealing, he will not desist from swearing falsely, when there is a doubt that the money might be his. Just as he is willing to make peace with the prohibition against stealing when there is a legitimate doubt, so too, he will make peace with the prohibition against swearing falsely when in doubt.