Order API: This API allows you to create, retrieve, update, and delete orders. When an order is placed, you can use this API to query the order object and send them to the supplier.
Paying the supplier would typically be handled outside of Shopify, according to the terms of your agreement with the supplier. Take a look at the above links and hopefully this will help your team to plan out the build of your app.
I want to describe to you the work scheme
Our sales channel via API receives data about the goods of the seller’s store, including prices. We place it on our side, a trading platform.
For our part, we place an order and place it in the seller’s store via the API. Here the question arises, how to correctly place an order via the API with prices lower than the prices of the seller’s store (dealer price)