'Amro bin Shu'aib narrated on the authority of his father, on the authority of his grandfather (RAA), That the Messenger of Allah (ﷺ) said:
"A slave who entered into an agreement to buy his freedom is still a slave as long as a Dirham of the agreed price remains to be paid." Related by Abu Dawud with a good chain of narrators. The full hadith is related by Ahmad and the three Imams. Al-Hakim graded it to be Sahih. (IT)
English reference (IT): Book 15, Hadith 1475 (IT)
Arabic reference (IT): Book 15, Hadith 1431 (IT)