What is the difference between a buy-back sale and a commodity sale used to obtain cash?

Some scholars do not distinguish between the issue of a buy-back sale and a commodity sale used to obtain cash?
That is incorrect. A commodity sale used to obtain cash is different. A commodity sale used to obtain cash is when you buy a commodity for a known term and sell it to people. The one who buys it does not sell it back to you; the one who buys it sells it to other people. If he sells it to you, that is a buy-back sale. But if he sells it to someone other than you, it is not a buy-back sale.
He sells it to someone other than you. He gets married, builds a house, or pays off a debt. He bought a car from him for one hundred thousand riyals and then sold it to someone else: there is nothing wrong with that.
In a buy-back sale, he sells it to him. But if he sells it to someone else, there is no usury or interest, and people have a need for this.