Is a sale in which the price and quantity of plastic products (polyolefin, stretch, PVC, printed) are agreed upon with the customer before purchasing the goods from the supplier and delivering them to the customer halal or haram?

If the interaction between the colleague and the client does not go beyond the initial agreement without completing the sale, then there is no harm. However, if a sale contract is concluded, this falls under the category of selling what one does not possess. This can be rectified through a Salaf (forward) contract if the commodity can be precisely described and the full price is paid at the time of the contract.