If someone gives a slave as a present or sold him under coercion, his deed is invalid

Narrated Jabir:

A man from the Ansar made his slave, a Mudabbar. And apart from that slave he did not have any other property. This news reached Allah's Apostle and he said, "Who will buy that slave from me?" So Nu'aim bin An-Nahham bought him for 800 Dirham. Jabir added: It was a coptic (Egyptian) slave who died that year.

حَدَّثَنَا أَبُو النُّعْمَانِ، حَدَّثَنَا حَمَّادُ بْنُ زَيْدٍ، عَنْ عَمْرِو بْنِ دِينَارٍ، عَنْ جَابِرٍ ـ رضى الله عنه ـ أَنَّ رَجُلاً، مِنَ الأَنْصَارِ دَبَّرَ مَمْلُوكًا، وَلَمْ يَكُنْ لَهُ مَالٌ غَيْرُهُ، فَبَلَغَ ذَلِكَ رَسُولَ اللَّهِ ﷺ فَقَالَ " مَنْ يَشْتَرِيهِ مِنِّي ". فَاشْتَرَاهُ نُعَيْمُ بْنُ النَّحَّامِ بِثَمَانِمِائَةِ دِرْهَمٍ. قَالَ فَسَمِعْتُ جَابِرًا يَقُولُ عَبْدًا قِبْطِيًّا مَاتَ عَامَ أَوَّلَ.