From Sahih Bukhari (Hadith 2562):
Narrated `Abdullah bin `Umar:Aisha wanted to buy a slave-girl in order to manumit her. The girl's masters stipulated that her Wala' would be for them. Allah's Messenger (ﷺ) said (to `Aisha), "What they stipulate should not stop you, for the Wala' is for the liberator
Reference: Bukhari 2562