Hi,

By default, Odoo does not automatically split replenishment between internal stock and purchase — it will either:

    Take from one source location (e.g., store B) until exhausted, or

    Create a purchase order if the source is empty, depending on the replenishment route.

Odoo doesn’t natively split the order automatically into partial internal transfer + purchase; this requires proper route and rules configuration.

Follow the steps:

    Check your routes

        Go to Inventory → Configuration → Routes.

        You need a route that combines:

            Take from another warehouse (internal transfer)

            Buy from vendor if insufficient stock

        Odoo calls this “Pull from another location then Buy” logic. The route must have two rules in sequence:

            Move from another warehouse (source: store B → destination: store A)

            Buy (make to order) if stock is insufficient

    Set up the replenishment rules

        Inventory → Operations → Replenishment

        For product X, set the minimum quantity at store A (e.g., 0) and the maximum quantity (e.g., 50).

        Set the route to the one created above (“Take from another warehouse then Buy”).

    Check push/pull rules

        The push/pull rules define how stock moves are executed:

            Pull rule: from store B → store A

            Pull rule: from vendor → store A

    Confirm warehouse configuration

        Make sure store A and store B belong to warehouses with proper inter-warehouse rules.

        Odoo uses inter-company and warehouse rules to generate the stock moves correctly.

Hope it helps

Assuming replenishment is set to manual, the 'To order' field can be modified to required to value.

In your example - set route to 'buy' and To order to 30 prior to triggering order

Post PO creation the To Order would set to 20 (remaining qty), change the route (Store B to Store A) and trigger order again.

If replenishment trigger is set to auto, only a single route with all of the qty can be triggered.

If the process is to always take from alternate store prior to ordering, you could consider modifying  the delivery route to accordingly.

eg configure rules as

Pull from Store A to customer (supply method take from stock if unavailable trigger another route)

Pull from Store B to Store A (supply method take from stock)

With the above route, 20 qty would be taken from Store B and balance 50 would be replenished with a buy