Is it considered a sin to invoke a curse upon a girl who has failed to deliver on a trust (amanah) she undertook, after six months have passed without her fulfilling it, and she continues to evade and breach the trust?

What this girl did, betraying trust and squandering rights, is Islamically forbidden. Allah has commanded the fulfillment of trusts and condemned their squandering, considering it a characteristic of hypocrites. You have the right to demand your right. It is permissible for you to criticize her or supplicate against her if she does not restore your right, based on the words of the Almighty: "Allah does not like the public utterance of evil, except by one who has been wronged." However, forgiveness and pardon are better.