Commentary for Avodah Zarah 143:7

In the first half of this baraita, the buyer brings the worker’s carrying the produce into his home. This does not count as “meshikhah” and therefore he does not acquire the produce, even if he sets a price.
However, if he unloads the produce, then he has performed “meshikhah.” In this case, if they set the price then neither can retract. This accords with Abaye. Setting the price is irrelevant if the buyer has not taken possession of the goods.