The Messenger of Allah ﷺ said, "When the buyer and seller disagree and there is no evidence between them, the word is that of the owner of the goods, or they may call off the transaction."
عن رسول الله ﷺ قال: إذا اختلف البيعان، وليس بينهما بينة فهو ما يقول رب السلعة، أو يتتاركان.
Reported by Abu Dawud (3511), Nasai (4652), Hakim (2/45), and Bayhaqi (5/332), all through the route of Abu Umays, from Abdul-Rahman bin Qays bin Muhammad bin Ashath, from his father, from his grandfather, that Abdullah bin Masud sold Ashath bin Qays some slaves from the fifth share, for twenty thousand dirhams. Abdullah then sent to him regarding their price, and he said, "I only took them for ten thousand." Abdullah said, "If you wish, I will tell you a hadith I heard from the Messenger of Allah ﷺ, I heard him say." Then he related the hadith.Hakim said, "Its isnad is sahih." Bayhaqi said, "This is a hasan, connected isnad, and it has been narrated through several routes with mursal chains, which, when combined, make the hadith strong by that combination." He then mentioned these mursal narrations. He said in Marifah (8/140), "The soundest isnad reported on this topic is the narration of Abu Umays from Abdul-Rahman bin Qays bin Muhammad bin Ashath, from his father, from his grandfather." He then mentioned the rest of the isnad.I say, but it contains Abdul-Rahman bin Qays, from whom only Abu Umays narrated, and for this reason he has been described as "unknown." Ibn Hajar said in Taqrib, "Acceptable," meaning when corroborated. It has indeed been corroborated, though only partially. Tirmidhi (1270) reported it from Qutayba, who said, Sufyan told us, from Ibn Ajlan, from Awn bin Abdullah, from Ibn Masud, who said, the Messenger of Allah ﷺ said, "When the two parties to a sale disagree, the seller's word is taken, and the buyer has the option [to proceed or withdraw]."Tirmidhi said, "This is a mursal hadith, Awn bin Abdullah did not meet Ibn Masud." I say, the hadith has other chains of transmission which I have mentioned in Minna Kubra (5/150).Khattabi said in his Maalim, "The scholars have agreed upon accepting this hadith, and that indicates that it has a basis, even though there is some discussion regarding its isnad, just as they agreed on accepting, 'There is no bequest to an heir.' Its isnad has what it has." Ibn Abdul-Hadi said in Tanqih (4/75), after citing it, along with Ibn Jawzi, through many routes, "What appears to be the case is that the hadith of Ibn Masud on this topic, taken together with all its routes, has a basis, indeed it is a hasan hadith that can be relied upon, though there is some variation in its wording, as you can see."The apparent meaning of the hadith indicates that when the seller and buyer disagree over some matter related to the contract, the seller's word is taken, or the buyer is given the choice between taking the goods at the price stated by the seller or leaving it. As for the jurists, they differed on this matter considerably, and I have discussed that in detail in Minna Kubra, so refer to it.