1720/1 - Abu Hurayrah (may Allah be pleased with him) reported that he heard the Prophet (may Allah’s peace and blessings be upon him) say: “Taking an oath may sell a commodity, but it takes the blessing away from the earnings.” [Narrated by Al-Bukhāri and Muslim]
1721/2 - Abu Qatādah (may Allah be pleased with him) reported that he heard the Prophet (may Allah’s peace and blessings be upon him) say: “Beware of excessive swearing in sale transactions, for it sells the goods and then erases the blessing.” [Narrated by Muslim]
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1) It is disliked to swear during buying and selling, even if the oath is true.
2) What matters is not the abundance of earnings from sale, but the blessing Allah puts in one’s sustenance, even if it is little.