The jurists of Madinah used to say: The foolish person (under guardianship) and the slave: the divorce of both of them is valid, and their freeing of slaves is void, except that the foolish person may free his umm walad (slave who bore him a child) if he wishes.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحجر / 11339