Reference for Bava Batra 207:1
פחות כל שהוא ינכה יתר כל שהוא יחזיר הא סתמא כהן חסר הן יתר דמי
AND HE GAVE [HIM] LESS, [EVEN IF ONLY BY] A FRACTION, [AN EQUAL SUM] IS TO BE DEDUCTED [FROM THE PRICE]. [IF] HE GAVE MORE, [EVEN IF ONLY BY] A FRACTION, IT IS TO BE RETURNED [TO HIM]. Thus [it is to be inferred that] had not [the expression 'measured by the rope'] been explicitly used [it would have been] just the same as if [the expression] 'more or less' [had been actually used]. Explain. [however], the concluding clause [which reads]: IF, HOWEVER, HE SAID, 'MORE OR LESS', THE SALE IS VALID EVEN IF HE GAVE [AT THE RATE OF] A QUARTER OF A <i>KAB</i> PER <i>SE'AH</i> LESS OR MORE. Thus [it is to be inferred that] had not [the expression, more or less'] been explicitly used [it would have been] just the same as if [the expression], 'measured by the rope' [had actually been used]! But, [one must conclude, that] nothing may be deduced from this [Mishnah].