Siman 311

We learned in Bekhorot (29): One who is suspected regarding bekhorot—according to the explanation of Rashi, that he is suspected of inflicting a blemish upon a bekhor, and according to the Rambam, that he is suspected of selling a bekhor as non-sacred meat—one may not purchase from him deer meat, as it is red and resembles calf meat, nor hides that are not tanned, lest they are from a bekhor. And even if it appears that the hide is from a female, we are concerned lest he cut the place of the male anatomy from the hide so that it appears as a female. However, tanned hides may be purchased from him, for since there is much labor involved in this, and he knows that if it is heard by a beit din he will lose everything as they will bury the hide, therefore, if it were from a bekhor, he would not exert himself this much. And the sages did not see fit in this case to penalize him regarding the permitted due to the prohibited, because the penalty regarding the prohibited alone is sufficient.

And likewise, one may not purchase wool from him, for perhaps it is from a firstborn animal. And even for whitening the wool, which involves some effort, nevertheless it is possible that he would exert himself in this due to the doubt, for perhaps it will not be known to the beit din. However, one may purchase from him dyed wool and felt, and also for the reason that has been explained: since this involves great effort, if it were from a firstborn animal, he would not have exerted such great effort, for the reason that has been explained.