Tafsir of Ash-Shu`ara' 26:182

This means with the correct scale. It is said that the Qustas is the steelyard (a type of balance); this is narrated from al-Hasan. Some consider it an Arabized word of Roman origin, meaning "justice," which is narrated from Mujahid. Others consider it Arabic, saying it is derived from al-qist (justice or equity), and its form is fi‘lāl—doubling the last letter—though this is irregular, as the letter is not doubled alone when separated by the lam. It is also said to be derived from qastasa, making it a quadriliteral word with the form fi‘lāl.

The intended meaning is the command to fulfill the weight completely and the prohibition against diminishing it, rather than a prohibition against adding to it. It is apparent that one is neither prohibited from adding to it nor commanded to do so in measuring and weighing. It is as if this is evidence that whoever adds to it has done well, and whoever does not, there is no blame upon him.

From Ibn Abbas (may Allah the Exalted be pleased with both of them) it is narrated that the meaning of "and weigh..." is: "Adjust all your affairs according to the scale of justice which Allah the Exalted has set for His servants." The most apparent interpretation, since He (glorified be He) equated it with "give full measure," is what has preceded.

Most of the seven canonical readers recited al-qustās with a damma on the qāf.