Beis Shammai say: One may not sell him a plowing cow during the seventh [year]. But Beis Hillel permit this, since he can slaughter it. One may sell him produce even during the planting season; and he may lend him his seah-measure even though he knows that he has a threshing-floor; and he may change maos into perutos for him, even though he knows that he has workers. But all of these are forbidden if they were stated explicitly.