The Quran

152

وَلا تَقرَبوا مالَ اليَتيمِ إِلّا بِالَّتي هِيَ أَحسَنُ حَتّىٰ يَبلُغَ أَشُدَّهُ ۖ وَأَوفُوا الكَيلَ وَالميزانَ بِالقِسطِ ۖ لا نُكَلِّفُ نَفسًا إِلّا وُسعَها ۖ وَإِذا قُلتُم فَاعدِلوا وَلَو كانَ ذا قُربىٰ ۖ وَبِعَهدِ اللَّهِ أَوفوا ۚ ذٰلِكُم وَصّاكُم بِهِ لَعَلَّكُم تَذَكَّرونَ

Muhammad Asad

and do not touch the substance of an orphan - save to improve it - before he comes of age."¹ And [in all your dealings] give full measure and weight,² with equity: [however,] We do not burden any human being with more than he is well able to bear;³ and when you voice an opinion, be just, even though it be [against] one near of kin.⁴ And [always] observe your bond with God:⁵ this has He enjoined upon you, so that you might keep it in mind.

I.e., after the orphan in one's charge has come of age, the former guardian may "touch" his property, legally, by borrowing from it or otherwise utilizing it with the owner's consent. The phrase rendered by me as "save to improve it" reads, literally, "in a manner that is best", which implies the intent of bettering it.
This refers metonymically to all dealings between men and not only to commercial transactions: hence my interpolation of "in all your dealings".
The meaning is that God does not expect man to behave with "mathematical" equity - which, in view of the many intangible factors involved, is rarely attainable in human dealings - but expects him to do his best towards achieving this ideal.
According to Razi, the phrase "when you voice an opinion" (lit., "when you speak") applies to expressing an opinion on any subject, whether it concerns one personally or not; but the subsequent reference to one's "near of kin" makes it probable that the above injunction relates, in particular, to the giving of evidence in cases under dispute.
See surah 2, note 19.