By the Bais Hora'ah
My brother-in-law sells esrogim and when I found a nice one, I asked the price and he told me that he would figure it out later. I insisted on making a kinyan kessef — the proprietary act of paying with money. I paid him a small amount, which thus constituted a valid kinyan, and we arranged that I would pay him the rest after we agreed upon the cost after Yom Tov. After Yom Tov I mentioned this arrangement to a friend and he questioned the validity of the kinyan since without an agreement regarding the price, there cannot be a kinyan.
Q: Did I fulfill the mitzvah or is my friend correct?
A: Your friend is referring to the halachah in Shulchan Aruch (C.M. 200:1) that states that a kinyan cannot be made without setting a price. This is repeated again (ibid. 7) when Shulchan Aruch rules that kinyanim are ineffective if the two parties did not decide a price per unit, per weight or per volume. The reason is that before the price is set, the two parties do not have semichus daas — reliable intent, since the other party may not agree to that price. When the price is fixed or if the two parties agree that the item will be sold in accordance with the appraisal of three experts (or even one expert — Sma 200:14), the transaction is binding since they have semichus daas even without the parties knowing the result of the appraisal.
An esrog is something that does not have a fixed price, since the price is determined by its quality and the timing of the sale. Accordingly, without agreeing upon a price, the kinyan should be invalid. Moreover, even to agree to accept the price of an expert is not effective since people will appraise esrogim differently depending on the customer’s taste, timing, etc., and thus an appraisal does not determine its value and thus does not generate a semichus daas. Therefore a kinyan cannot be made without an agreement on the price (Teshuvos V’hanhagos 3:189).
Some authorities assert that the prerequisite for setting a price for the kinyan to be valid is limited to circumstances where the object to be sold will remain intact. The buyer takes the object to decide whether he wants to keep it and if not, he will return it. In such a circumstance, since there is no agreement about the price, there is no semichus daas for the kinyan. If the object cannot be returned intact, e.g., it is a food that will be consumed, there is most certainly semichus daas even though the price was not set, and thus a valid kinyan could be made since the object cannot be returned intact. Therefore, once the food was consumed, the seller may claim the amount the item would cost in the market.
An esrog is in the same category as food since it loses most of its value after the first day of Sukkos, and after the chag it is worth less than an apple. Accordingly, it is evident that the seller intends to sell it completely and collect what he could have collected for the esrog before Yom Tov (see Ateres Moshe, O.C. 230 and Ginzei Chaim, O.C. 306:5).
Since you and your brother-in-law agreed to determine the price later, its price is determined by what its value was before Yom Tov, despite the difficulty involved in determining its exact value, as mentioned above (see Erech Shai 182:12). Moreover, since your transaction was with your brother-in-law, it is clear that there is a semichus daas on the kinyan and the two of you intended to reach an agreement in the future.
Obviously, it is preferable to reach an agreement before Sukkos rather than waiting until Chol Hamoed or after Sukkos.