Musannaf Ibn Abi Shaybah

It is narrated from Ibrahim that if a slave says to his master: Sell me to so-and-so for such and such an amount, and I will owe you five hundred dirhams, or if someone comes and stands as guarantor and says: Sell your slave to so-and-so, you will have five hundred dirhams, is such a statement correct? He replied: This condition is invalid.

Hazrat Sha'bi (may Allah have mercy on him) says that it is not permissible.