«وَفَاءِ الْكَيْلِ أَوْ بَخْسِهِ»
Whether completing the measure or diminishing it.
Wafāʾ al-kayl suggests giving a full measure; bakhs means diminishing what is due. The pair is first intelligible in ordinary commerce, then carries over into human relations.
Literal case: giving someone the full amount of grain promised.
Figurative case: fulfilling obligations of trust and loyalty in friendship.
This is a representative analogy: the known scene of commerce explains a less tangible moral relationship.