Demai Chapter 3, Mishnah 2. This mishnah deals with the obligation of the chaver - a person known to be trustworthy and careful about separating tithes - to make sure that nothing leaves his hands from which tithes have not been separated, whether by way of sale or by way of gift, and whether the produce was bought from an am ha'aretz or is his own, as we will see.
"HaRotzeh lachtoch alei yarak":
Someone carrying vegetables who wishes to cut off some of the leaves in order to lighten his load and throw them away - "lo yashlich ad sheye'aser": he may not throw the leaves away before separating tithes from them, out of concern that an am ha'aretz will find them and eat them.
"HaLokeach yarak min hashuk venimlach lehachzir":
A chaver who bought vegetables in the market and then changed his mind and wants to return them: he has already performed meshichah, which is an act of acquisition, and now he wishes to back out. The mishnah says "lo yachzir ad sheye'aser" - since the vegetables are already his, the responsibility rests on him to separate tithes from them before returning them.
"She'eino mechusar ela minyan":
This phrase describes the state of the purchase: the only thing still lacking is counting the number of items he took, while the act of acquisition itself has already been done - whether by meshichah or by hagbahah. All that remains is for him and the seller to settle the account according to the count; but as far as ownership is concerned, he is already the owner of the vegetables.
"Hayah omed velokeach":
What is the ruling when he is standing in the market in the middle of choosing and inspecting, but has not yet performed an act of acquisition? He examined the item he wanted to buy, "vera'ah acher yafeh mimenu" - he saw another portion that was nicer. In this case "mutar lehachzir", and the reason is: "shelo mashach" - because he performed no act of acquisition, neither meshichah nor hagbahah, and he was never the owner. And since he was never the owner, he has no obligation to separate tithes before returning it.
In summary: the mishnah presents three situations:
"HaRotzeh lachtoch alei yarak": he may not throw the leaves away until he tithes, out of concern that an am ha'aretz will find them and eat them.
"HaLokeach yarak min hashuk venimlach lehachzir": he may not return them until he tithes, since he already performed meshichah and acquired them, and the only thing lacking is the count.
"Hayah omed velokeach vera'ah acher yafeh mimenu": he may return it without tithing, since he performed neither meshichah nor hagbahah, and never became its owner at all.
The principle emerging from the mishnah: the obligation to separate tithes depends on ownership - once the item has become his, he may not let it out of his hands before separating tithes from it.