An-Nisa' - 4:2

وَآتُواْ الْيَتَامَى أَمْوَالَهُمْ وَلاَ تَتَبَدَّلُواْ الْخَبِيثَ بِالطَّيِّبِ وَلاَ تَأْكُلُواْ أَمْوَالَهُمْ إِلَى أَمْوَالِكُمْ إِنَّهُ كَانَ حُوبًا كَبِيرًا
[4:2]
And give to the orphans their property, and do not substitute worthless (things) for (their) good (ones), and do not devour their property (as an addition) to your own property; this is surely a great crime.
To orphans restore their property (When they reach their age), nor substitute (your) worthless things for (their) good ones; and devour not their substance (by mixing it up) with your won. For this is indeed a great sin.
Give unto orphans their wealth. Exchange not the good for the bad (in your management thereof) nor absorb their wealth into your own wealth. Lo! that would be a great sin.