Bava Kamma 73A

משום פסידא דלקוחות.

It is due to the potential monetary loss for purchasers, whose acquisitions had been validated by these witnesses between the time of the witnesses’ first testimony and when they were rendered conspiring witnesses. If the disqualification of the witnesses were applied retroactively, as by right it should, all these transactions would be nullified, which would cause a loss to these purchasers.

מאי בינייהו? דאסהידו ביה תרי לחד, ותרי לחד. אי נמי דפסלינהו בגזלנותא.

The Talmud asks: What is the practical difference between these two explanations of Rava’s opinion? After all, according to both explanations Rava did not apply the disqualification of conspiring witnesses retroactively. The Talmud explains that there is a difference in a case where two witnesses testify about one of the witnesses that he was not at the scene of the supposed crime, and two other witnesses testify about the other one witness in a similar manner. Alternatively, there is a practical difference between the two explanations in a case where two witnesses disqualified the first set of witnesses by testifying that they had once committed robbery and are therefore unfit to give testimony.

להך לישנא דאמרת משום חידוש ⁠– ליכא. להך לישנא דאמרת משום פסידא דלקוחות ⁠– איכא.

The Talmud elaborates: According to this version in which you say that Rava’s rejection of retroactive disqualification was because it is a novelty, in these two circumstances there is no novelty, and therefore he would agree that the disqualification should be retroactive. According to that version in which you say that Rava’s concern was due to a potential loss for purchasers, in these two circumstances there is a concern for a potential loss for purchasers. Consequently, in these circumstances as well Rava would reject retroactive disqualification.