Yes. The Quran commands returning trusts to their owners (4:58) and forbids betraying them (8:27). This includes money left with you, secrets, and the duties of a job. Delaying payment of a debt when you are able to pay is also a form of injustice.
Certes, Allah vous commande de rendre les dépôts à leurs ayants-droit, et quand vous jugez entre des gens, de juger avec équité. Quelle bonne exhortation qu'Allah vous fait! Allah est, en vérité, Celui qui entend et qui voit tout.
O vous qui croyez! Ne trahissez pas Allah et le Messager. Ne trahissez pas sciemment la confiance qu'on a placée en vous?
This is general information, not a fatwa. For your own situation, ask a qualified scholar.
Questions about halal and haram
Is it haram to betray a trust?
Yes. The Quran commands returning trusts to their owners (4:58) and forbids betraying them (8:27). This includes money left with you, secrets, and the duties of a job. Delaying payment of a debt when you are able to pay is also a form of injustice.