Is the sale of a debt owed by a Mukatab (contracted slave) based on his installments valid?

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The sale of the debt owed by a Mukatab based on his stipulated installments is invalid. This view is held by Abu Hanifa, Al-Shafi'i, and Abu Thawr. The basis for invalidity is that this debt is unstable (ghayr mustaqirr) because it is subject to cancellation should the Mukatab become unable to pay. Furthermore, the master lacks the authority to compel the slave to pay or obligate him to secure the payment, similar to a condition for a gratuitous donation. The sale is also invalid because the debt has not been fully taken possession of, and the Prophet, peace be upon him, prohibited the sale of what has not been possessed. If such a sale occurs, it is void, and the buyer cannot demand the payment from the Mukatab nor reclaim the price from the seller if it was already paid.