I have a similar use case as the following post: https://www.odoo.com/forum/help-1/i-have-5-units-of-a-product-and-a-sales-order-for-10-can-odoo-take-the-units-from-stock-first-then-create-a-purchase-order-for-the-remaining-quantity-205598
The difference is that I use a two-step receipt. Does anything change? Do I need a reordering rule?