The ruling on installment sales without a fixed price

Question:
The issue of installments involves companies that offer installments by saying: "The car is one hundred thousand. If you leave it for one year, we will take one hundred and ten from you; for two years, one hundred and twenty; for three years, one hundred and thirty?"

Answer:
There must be a fixed amount; this way is not permissible. There must be a fixed amount for three or four installments. If it is fixed, there is no harm: so much each year, so much each month...
Q: If he says to him: "For the year, a specific percentage?"
The Shaykh: There must be a fixed amount.
Q: How do you mean by "fixed amount"?
The Shaykh: Forty thousand, ten thousand each year, fixed; or fifty thousand, ten thousand each year.
Q: Some companies now take a percentage for the year, saying eight and a half percent for the year, or ten percent for the year?
The Shaykh: The point is that they agree on a known amount; that is the point, whether it is each year, each month, or every two months.