משנה על דמאי 6:2

Bet Shammai says: one must not sell him a plowing cow in the seventh year. But Bet Hillel permits, since he may slaughter it. One may sell him produce even at sowing time. And one may lend him a seah measure, even though he knows that he has a threshing-floor. One may exchange coins for him, even though he knows that he has workers. But all these things, if he expresses [that they will be used for unlawful purposes], then they are forbidden.