The Prophet ﷺ said, "A slave under a contract of manumission remains a slave as long as a single dirham of his contract remains unpaid."
حدثنا هارون بن عبد الله حدثنا أبو بدر حدثني أبو عتبة إسمعيل بن عياش حدثني سليمان بن سليم
عن النبي ﷺ قال المكاتب عبد ما بقي عليه من مكاتبته درهم
Its chain of transmission is hasan. Bayhaqi narrated it, 10/324, from the hadith of Abu Dawud with the same wording, and see the following hadith.