Commentary
Breaking Pledges is Forbidden
The word: عَھد ('ahd) is inclusive of all transactions, pacts and pledges the responsibility of which has been assumed verbally, whether one does or does not swear by it, and whether it relates to doing something or not doing it.
In fact, these verses are the explanation and conclusion of previous verses. The previous verses carried the command to do justice and to be good. The sense of justice there includes the fulfillment of pledges as well. [ Qurtubi ]
After having given a pledge or made a contract with someone, breaking it is a major sin. But, in the event it is broken, there is no Kaffarah (expiation) fixed for this purpose. Instead, it brings the punishment of the Hereafter on the offender. According to Hadith, the Holy Prophet ﷺ said that a flag shall be planted on the back of the pledge-breaker on the Day of Resurrection which will become the cause of his or her disgrace there.
Similarly, acting contrary to what one has sworn by is also a major sin. In the Hereafter, it is full of unimaginably evil consequences while in the present world too there are its particular situations in which Kaffarah (expiation) becomes necessary. [ Qurtubi ]