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.
Allah befiehlt euch, anvertraute Güter ihren Eigentümern (wieder) auszuhändigen und, wenn ihr zwischen den Menschen richtet, in Gerechtigkeit zu richten. Wie trefflich ist das, womit Allah euch ermahnt! Gewiß, Allah ist Allhörend und Allsehend.
O die ihr glaubt, verratet nicht Allah und den Gesandten, und handelt nicht verräterisch in Bezug auf die euch anvertrauten Güter, wo ihr wisset!
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.