Language:
كتاب الخصومات
44  
Khusoomaat
(3)
Chapter: If somebody sells a thing for a weak-minded person and pays him the price, and advised him to utilize it in a proper way and not to spoil it, and he spoils it, he can stop him from doing so, for the Prophet had forbidden the wasting of the property. The Prophet ordered the person who said that he had always been cheated in buying, to say, at the time of the deal, "No cheating!" The Prophet did not take his money (of the person who sold his slave, because he had not proved foolish).
(3)
باب مَنْ بَاعَ عَلَى الضَّعِيفِ وَنَحْوِهِ فَدَفَعَ ثَمَنَهُ إِلَيْهِ، وَأَمَرَهُ بِالإِصْلاَحِ وَالْقِيَامِ بِشَأْنِهِ، فَإِنْ أَفْسَدَ بَعْدُ مَنَعَهُ، لأَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ نَهَى عَنْ إِضَاعَةِ الْمَالِ، وَقَالَ لِلَّذِي يُخْدَعُ فِي الْبَيْعِ: «إِذَا بَايَعْتَ فَقُلْ لاَ خِلاَبَةَ». وَلَمْ يَأْخُذِ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ مَالَهُ
Sahih al-Bukhari 2415

Narrated Jabir:

A man manumitted a slave and he had no other property than that, so the Prophet (ﷺ) canceled the manumission (and sold the slave for him). Nu'aim bin Al-Nahham bought the slave from him.

حَدَّثَنَا عَاصِمُ بْنُ عَلِيٍّ، حَدَّثَنَا ابْنُ أَبِي ذِئْبٍ، عَنْ مُحَمَّدِ بْنِ الْمُنْكَدِرِ، عَنْ جَابِرٍ ـ رضى الله عنه ـ أَنَّ رَجُلاً، أَعْتَقَ عَبْدًا لَهُ، لَيْسَ لَهُ مَالٌ غَيْرُهُ، فَرَدَّهُ النَّبِيُّ صلى الله عليه وسلم، فَابْتَاعَهُ مِنْهُ نُعَيْمُ بْنُ النَّحَّامِ‏.‏
Reference : Sahih al-Bukhari 2415
In-book reference : Book 44, Hadith 6
USC-MSA web (English) reference : Vol. 3, Book 41, Hadith 598
  (deprecated numbering scheme)