All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
If someone gives you a sum of money to keep as a trust and you dispose of it without his permission, you have committed a sin and breach of trust, even if you know that you are able to give it back and even if you are very wealthy. It is impermissible to dispose of the trust without the permission of its owner.
Ibn ʻUthaymeen said:
"If someone is given a few dirhams to keep as a trust, it is impermissible for him to dispose of this money without the owner's permission or dispose of it for his personal benefit. If he does so, it is considered a breach of trust and he is subject to the verse which reads (what means): {O you who have believed, do not betray Allah and the Messenger or betray your trusts while you know (the consequence).} [Quran 8:27] It is also a violation of the verse: {Indeed, Allah commands you to render trusts to whom they are due...} [Quran 4:58]" [Fataawa Noor ʻala Ad-Darb]
Allah knows best.