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.
Alá os ordena que restituyáis los depósitos a sus propietarios y que cuando decidáis entre los hombres lo hagáis con justicia. ¡Qué bueno es aquello a que Alá os exhorta! Alá todo lo oye, todo lo ve.
¡Creyentes! ¡No traicionéis a Alá y a Enviado! ¡No traicionéis, a sabiendas, la confianza puesta en vosotros!
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.