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.
Deus manda restituir a seu dono o que vos está confiado; quando julgardes vossos semelhantes, fazei-o eqüidade. Quãoexcelente é isso a que Deus vos exorta! Ele é Oniouvinte, Onividente.
Ó fiéis, não atraiçoeis Deus e Mensageiro; não atraiçoeis, conscientemente, o que vos foi confiado!
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.