Is failing to fulfill a promise to one's mother to pay traffic fines considered a lie or a breach of covenant with Allah or with one's mother, and does this entail expiation?

If you promised your mother to pay the fine and were sincere in your intention but became unable to do so, there is no sin upon you, nor is an expiation for an oath required. However, if you promised her and intended not to pay, this is a forbidden lie, from which you must repent by ceasing the act, regretting it, and resolving not to return to it.