Muhammad bin al-Muthna - al-Zaman محمد بن المثنى أبو موسى - الزمن · The Book of Agriculture

"I said to 'Ata': 'What if I hire a slave for a year in return for his food, and for another year, in return for such and such?' He said: 'There is nothing wrong with that, and you may stipulate your conditions of hiring even for a few days.' 'How about if I make a deal to hire him when part of the year has passed?' He said: 'Do not hold me to account for what has passed.'"

It was narrated from Jabir that the Prophet forbade Al-Muhaqalah, Al-Muzabanah, Al-Mukhabarah and exceptions when selling, unless they were well-defined.