Sunan Ibn Majah Book 19, Hadith 15
“Umm Salamah freed me but stipulated that I should serve the Prophet (ﷺ) as long he lived.”
“Whoever becomes the master of a Mahram relative, he becomes free.”
“Whoever frees his share of a slave or part of his share, must pay from his wealth if he has any wealth if he has any wealth (in order to buy the rest of the slave's freedom). If he does not have wealth, then the slave should be asked to work for the price (of his freedom), without that causing him too much hardship.”