Is it considered a sin to supplicate that the betrothed girl be one's lot, for fear that this might lead to wishing for the failure of her engagement?

Du'a (supplication) is an act of worship. It may become obligatory, forbidden, or disliked. It is not permissible to supplicate for something that involves the loss of Muslims' rights or injustice to others. Therefore, it is not permissible to supplicate that a Muslim man's wife or fiancée becomes the supplicant's share, because that entails supplicating for the dissolution of an engagement or an existing relationship, which is an injustice to others and an attempt to cause harm unjustly. Allah may answer an unjust supplication to punish the one supplicated against for his sins or to raise his ranks, while the supplicant is punished for his wrongful supplication.