The ruling on installment sales when the installments and terms are known

Question: I sold a car to one of my friends for the sum of 40,000 riyals, to be paid in monthly installments of 2,000 riyals each. This friend is going through a financial crisis, so he asked me to sell it at the dealership—since the car is still in my name. I took a promissory note for the price and stipulated that my friend be present with me at the time of sale to receive the price, and that the sale be with his consent. He agreed, and my car was sold at the dealership for 27,000 riyals. He then received the price of the car, with which he paid off some of his debts.
I did not have any money to lend, and I had nothing but the car he had previously bought from me—as I mentioned—so much so that I had no desire to sell it. Is there any riba in this transaction of mine? And is installment sale permissible, knowing that my car cost me 35,000 riyals?

Answer: There is no harm in installment sale if the installments are known and the terms are known; due to the generality of His saying (peace and blessings be upon him) : [al-Baqarah: 282]. Thus, Allah has permitted debt to a specified term.
So if the installments are clear and known, and the terms are known, there is no harm, just as you did with your friend: 2,000 each month, known, and the total is known (40,000). There is nothing wrong with this, if you sold the car while it was in your ownership, under your possession and control. There is no harm in that.
As for your taking charge of this, you are doing a favor, and this falls under the category of agency. You are doing a favor and will be rewarded as long as you did it for the sake of Allah.