When a product uses AVCO (Average Cost), Odoo always values returns at the current average cost, not at the original purchase price. This is expected behavior; it is how the AVCO method works by design.

Under AVCO, inventory valuation is based on the weighted average cost of all quantities on hand. When you return a product, Odoo uses that current average cost to maintain consistency in stock valuation. It does not revert to the original purchase price of the specific unit.

If you require returns to be valued at their original purchase cost, AVCO is not suitable. You should use FIFO instead, because FIFO tracks layer-based costs and returns can reference the original valuation layer.