What is the ruling on an elderly person selling their house for less than its market value, on condition that they remain in it until their death, in addition to the buyer paying an agreed-upon monthly income to the seller until their death?

This transaction contains two impermissible elements: the unknown duration of the seller's occupancy of the house, and the unknown price due to not knowing the number of monthly rent installments. To rectify it, a known duration for the seller's occupancy must be agreed upon, and the total price of the house must be determined.