If I sell someone an item (e.g. an orange), and I attach a condition (e.g. "you may not make orange juice"), is it successfully binding? What happens if that someone does violate the condition (e.g. makes orange juice)?
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The concept is the subject of interesting debate between the authorities, particularly nowadays in the realm of intellectual property. There's a good writeup of some important sources on pages 19-24 of this YU Shavuot-To-Go packet by R' Josh Flug.
On the first day of Sukkot, if you find a Jew that doesn't have the four species, the solution is to give yours on condition that he returns it after he fulfills the mitsva. If he doesn't intend to do that, the kinian he did in first place is not valid.