The mishnah in the fifth chapter of Maseches Sheviis deals with the question of selling and lending to a person who is suspected of not keeping the laws of shemittah: which items may be sold or lent to him, even though there is a concern that he will use them to commit a transgression.
The dispute between Beis Shammai and Beis Hillel about selling a cow:
"Beis Shammai omrim: lo yimacher lo parah choresheth ba'sheviis" - one may not sell a plowing cow to a person under suspicion, that is, a cow capable of plowing, out of concern that he will use it to plow a field during the sheviis year.
"U'Beis Hillel matirin, bishvil shehu yachol l'shochtah" - since it is possible that he will slaughter the cow and use it for food. And as long as there is a possible attribution, meaning a plausible explanation under which his act involves no transgression, Beis Hillel hold that one may sell him the item, even though there is a concern or suspicion that he will misuse it and use it against halachah.
Beis Shammai reject this attribution: it is not the way of people to slaughter a cow fit for plowing, since a cow capable of plowing is worth more, and therefore one cannot attribute the purchase to slaughter.
Additional cases that the mishnah permits on the strength of attribution:
"Mocher lo peiros afilu bish'as hazera" - one may sell him produce and grain, including cereals, even at the season when people normally plant, even though there was room to be concerned that he would take the seed and plant it instead of using it for food. And nevertheless it is permitted, since one can attribute that he will use it for eating.
"U'mash'il lo se'ah, af al pi shehu yodea sheyesh lo goren" - a se'ah is a measure ordinarily used for measuring grain, and there is room to be concerned that he will use it in the process of growing and producing grain; and even though he knows that the man has a goren, a pile of grain that grew in a forbidden manner - since the se'ah can be used for other purposes, it is permitted.
"U'poret lo ma'os, af al pi shehu yodea sheyesh lo po'alim" - one may exchange coins for him, even though he knows that he has workers whom he employs in a way that is not proper, since one can attribute that he will use the money for some other purpose.
"V'chulan b'feirush - assur" - in each of these examples, if the suspected person stated explicitly that he intends to use the item for a forbidden purpose, it is forbidden to sell him and forbidden to lend him any of these things.
In summary: in this mishnah we learned the principle of attribution in selling to one suspected regarding sheviis: according to Beis Hillel, whenever one can attribute the item to a permitted use, the sale is permitted, and therefore we sell produce even at planting season, lend a se'ah even when we know he has a goren, and exchange coins even when we know he has workers. Beis Shammai reject the attribution in the case of a plowing cow, since it is not the way to slaughter a cow fit for plowing. And in all the cases, once the suspected person states explicitly that his intent is for a forbidden purpose, it is forbidden.