How should one deal with a manager who broke his promise to provide a suitable job, knowing that he hired someone unqualified?

Fulfilling a promise is recommended by the majority of scholars, and it becomes obligatory if breaking it results in harm to the one promised. If the one who made the promise was able to fulfill it but then broke it, and harm resulted from that, he is sinful and must repent and rectify the matter if possible; otherwise, he must repent and seek forgiveness. If no harm resulted, there is no sin.

It is not permissible to accuse anyone of shortcomings without proof, or of adultery except with the testimony of four witnesses. Nor is it permissible to spread such accusations, as Allah says: "Indeed, those who like that immorality should be spread [or publicized] among those who have believed will have a painful punishment in this world and the Hereafter. And Allah knows and you do not know." [An-Nur: 19].

It is not permissible to abandon a relative if the failure [to fulfill a promise] was due to inability. However, it is permissible to abandon him if the failure occurred despite being able, and if abandoning him serves as a deterrent. Thus, abandonment depends on what serves the greater good and what prevents harm.