1
In this case, one may assume that both want to complete the deal at three dinars, and they did not intend to vow but only exaggerated for purposes of bargaining.
2
GEMARA: The mishna states: The Sages dissolved four types of vows. Rabbi Abba bar Memel said to Rabbi Ami: You said to us in the name of Rabbi Yehuda Nesia: Who is the tanna who taught this mishna of four vows? It is the opinion of Rabbi Yehuda, who said in the name of Rabbi Tarfon: With regard to two people who entered into a wager, and each declared that he would become a nazirite if the other was right, then actually neither of them becomes a nazirite, because naziriteship is determined only by explicitness of intent. One cannot become a nazirite unless he vows clearly and with certitude. Here too, since the intent of the wagering parties was not actually to vow, the halakha is that the vow is invalid.
3
Rava said: You can even say that the mishna is in accordance with the Rabbis, who disagree with Rabbi Yehuda. Does the mishna teach: Both wanted the price to be three dinars? It teaches: Both want, in the present tense, demonstrating that they would have been satisfied with that price from the outset and never intended to vow, so the vow is not binding.
4
Ravina said to Rav Ashi: If the seller said to the buyer in the form of a vow that the price must remain more than a sela, and the other declared in the form of a vow that the price must remain less than a shekel, what is the halakha? Is this a vow, where each stands firmly in his position? Or perhaps this is also merely for encouragement?
5
Rav Ashi said to him: We already learned about a similar case: If one was importuning another that he should eat with him and the other refused and said to him: Entering your house is konam for me, or: A drop of cold water is konam for me, and for that reason I will not taste it with you, it is permitted for him to enter his house and drink cold water since he intended it as a vow only for the purpose of eating and drinking a large amount but did not mean literally that he would not drink anything.
6
The Gemara asks: And why is this permitted? But he said: A drop of cold water, so how can you say that the vow is only with regard to drinking a large amount? Rather, it must be that a person speaks this way in exaggerated terms but does not mean literally what he says. Here also, in the case of the buyer and seller, a person speaks this way. He exaggerates and does not intend the literal meaning of his words, even though he was quite precise in his wording.
7
Ravina said to him: