It is forbidden to deceive someone (geneivat da’at).[1] Although you presumably are aware of this, your question is probably premised on the mitigating circumstances: 1) You cited the price with honest intentions. 2) By the time you realized the error, he had already agreed to the price, indicating that the price was reasonable and worthwhile for him.[2]
We will divide our discussion according to whether you found out before or after the transaction was halachically complete.[3]
Geneivat da’at applies even when one said nothing false, but just acted in a manner that created a misconception.[4] The gemara[5] says that acting in a manner that gives a false impression is considered a violation of the requirement to distance oneself from a lie, even if one said nothing. Thus, if you complete the transaction after realizing that you gave false information, then you are deceiving the seller at that point. Going through with the transaction without correcting the mistake is not much different from lying in the first place.
What if the transaction was already completed before you discovered your mistake? The rules of ona’ah (mispricing) deal with situations in which the transaction already occurred and the question is whether the sale should be voided or the price differential returned. The general rule is that if the difference between the actual sales price and the proper one is less than one sixth, one does not have to return the difference.[6] In your case, if we could determine that the price was not off by that much from the range of normal prices, there ostensibly should be no consequences after the fact due to your questionable discount. Furthermore, when the “victim” of the unfair price was told the proper price but still agreed to the “wrong” one, the agreement stands as is.[7] Since the proprietor arguably knew the costs and prices in his field, he might be equivalent to a buyer who was told the real price and still agreed.[8]
Nevertheless, there is a fundamental distinction between the regular rules of ona’ah, which deal with appraisal of value, and a factual mistake. The leeway of a sixth given for ona’ah is based on the fact that setting an exact price is an inexact science of, making modestly differing prices marginally legitimate.[9] However, if someone gives false information regarding something exact – such as measurements of size, weight, or number – ona’ah applies even for a difference of less than a sixth and even for objects that are excluded from the standard laws of ona’ah.[10] As the Shulchan Aruch[11] explains, this is considered a kinyan b’ta’ut, a transaction made under false pretenses. In the same vein, if the price in your case was based on an exact fact – i.e., the price you said was quoted by a different vendor – ona’ah applies even if it is off slightly. The fact that the proprietor agreed to a low price reflects a calculated concession on his part, but the parameters of that concession are predicated on the buyer’s presenting accurate competing quotes. Additionally, in the case of a kinyan b’ta’ut, it makes no difference whether the one who provided the wrong information did so intentionally or accidentally.[12]
An important distinction is crucial here. Sometimes a buyer or seller will try to make a deal look better through information he provides (e.g., “the going rate is X, but I am giving you a discount”). Untruths about the going rate are not always grounds for invoking ona’ah if other factors are missing.[13] However, if a seller sets the price strictly based on, for example, the price he gives other customers, and he lied about that price, ona’ah does apply in spite of the fact that the price he sets is objectively reasonable.[14] In such cases, one has to analyze the language of the agreement: Was the price they agreed upon one they could have arrived at without inaccuracies, and the false information was simply an attempt to convince the buyer, or was the price for others the basis for the price here?[15]
Let us apply this reasoning to your case. The way the seller worded your discussion on the price (“I will beat any price…”) indicates direct linkage to the quote you presented. Therefore, even if the transaction had been completed before you realized the mistake, you would have to initiate the return of any price distortion created by your mistake.[16]