Is it permissible for "A" to pay "S" an amount of 1000 for a commodity that "S" bought from "P" for 2000 on credit, knowing that its cash price from "P" is 1500, such that "A" receives the commodity from "P"?

It is permissible to buy a commodity on credit for more than its current price, and it is permissible to sell it for less than its purchase price, provided that the buyer is not the original seller (this is the issue of tawarruq). However, selling it back to the first seller for less than its price is not permissible, because it is the prohibited inah sale.

Therefore, if "S" will lend the amount to "A" so that "A" can buy the commodity from "Z", there is no harm. But if "Z" is the one who will buy and then sell to "A", then it is not permissible because it is an inah sale.