Volume 3, Book 46, Number 720:
Narrated Abu Musa (radiallaahu `anhu):
Allah’s Apostle (sallallaahu `alayhi wasallam) said, “He who has a slave-girl and educates and treats her nicely and then manumits and marries her, will get a double reward.“
Another similar narration:
The Prophet SAWS (peace and blessings of Allaah be upon him) said: “There are three who will be given a double reward: a man from among the people of the Book who believed in his Prophet, then lived to see the Prophet (S) and followed him and believed in him– he will have a double reward; and a slave who fulfills his duty towards Allaah and towards his master – he will have a double reward; and a man who had a slave woman whom he fed and fed her well, and taught her and taught her well, then he set her free and married her – he will have a double reward”
Narrated by al-Bukhaari (97) and Muslim (154).
The act of educating a slave-girl and treating her nicely is rewarded.. and marrying her after freeing her gets the owner double reward.. because this is kind treatment multiplied by two.
P.S. a man cannot marry his slave because ownership is stronger than wedlock. Read more here.