Am I sinful for breaking off an engagement after making a covenant with Allah Almighty to marry a righteous man?

A covenant with Allah is a form of oath and vow. Therefore, whoever makes a covenant with Allah to perform an act of obedience, it becomes obligatory upon him to perform it. If he does not perform it, he must offer expiation for an oath (kaffarat yamin).

The expiation for an oath is feeding ten poor people with the average food you feed your own families, or clothing them, or freeing a slave. If one cannot find these, then fasting for three days. This is based on the Almighty's saying: (Allah will not impose blame upon you for what is unintentional in your oaths, but He will impose blame upon you for what you intended in your oaths. So its expiation is the feeding of ten poor people from the average of that which you feed your [own] families or clothing them or the freeing of a slave. But whoever cannot find [or afford it] - then a fast of three days. That is the expiation for your oaths when you have sworn. And guard your oaths. Thus does Allah make clear to you His verses that you may be grateful.) Al-Ma'idah/89.