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.
Hakika Mwenyezi Mungu anakuamrisheni mzirudishe amana kwa wenyewe. Na mnapo hukumu baina ya watu mhukumu kwa uadilifu. Hakika haya anayo kuwaidhini Mwenyezi Mungu ni mazuri sana. Hakika Mwenyezi Mungu ndiye Mwenye kusikia, Mwenye kuona.
Enyi mlio amini! Msimfanyie khiana Mwenyezi Mungu na Mtume, wala msikhini amana zenu, nanyi mnajua.
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.