פירוש על שבועות 4:11

הם יודים לו דות ד מפי ד וכו' פטורים – for if they testified about him, their testimony would not be worth him anything.

If he said to two [persons]: “I adjure you, so-and-so and so-and-so, that if you know any testimony for me you should come and bear testimony for me”: [And they replied,] “We swear we know no testimony for you”, and they did know testimony for him, [but it was evidence of] one witness from the mouth of another witness; or if one of them was a relative or [otherwise] ineligible [as a witness], they are exempt. This mishnah teaches that if the witnesses who swore that they do not know testimony did know testimony but that the testimony would not have been accepted in the court, they are exempt for having sworn falsely. The reasons that their testimony might not have been accepted are: 1) the testimony was hearsay (see Sanhedrin 4:5); 2) they were related to one of the litigants (see mishnah one of this chapter); 3) they were otherwise unfit to testify (ibid.). Since their testimony would not have been sufficient to render a verdict in any case, they are not liable for having sworn falsely.