Is supplicating for marriage to a specific person—who was rejected due to social circumstances—a sin, or does it not contradict the belief in Allah's divine decree and destiny?

There is no objection to supplicating for marriage to a specific person, as this does not contradict contentment with divine decree. Rather, supplication is one of the greatest permissible means. It is preferable to entrust the matter to Allah Almighty when supplicating, for Allah knows what is good for His servant. The supplication of a Muslim is always answered in one of three ways: the supplication is hastened, or it is stored for him in the Hereafter, or an equivalent evil is averted from him. One must be certain that all of Allah's decrees are good, for Allah alone knows what is best for His servant.