Related על דמאי 3:2

[With respect to] one who buys vegetables from an outdoor market, behold, this one spends the entire day [deciding what produce to] select, and we need not be concerned; [however] it is forbidden for him to make up his mind [while holding the produce in one's hand but before purchasing it, see Dem. 3:2], [since] he cannot return it, because he needs to tithe, and he cannot tithe it, because [if he changes his mind and returns the produce to the seller] it will be short some quantity [because he has tithed it], and he cannot buy just one item [constituting priestly portion, while returning the rest (per Lieberman)], lest another [buyer] comes and buys [the returned portion], and [unwittingly] tithes already-tithed produce in exchange for untithed produce [see, e.g., Dem. 5:3].