inventory_adjust_quantities

Move stock by a relative amount: delta of 5 adds five, -5 removes five. This is not a way to set an absolute count. Every change must also carry change_from_quantity, the count you expect to be overwriting — read it with variant_inventory_level immediately before — so two adjustments racing each other cannot both win. committed is never writable, and on_hand cannot be named here although adjusting available moves it by the same delta. The result reports the deltas applied and leaves quantity_after_change null, so read the level back for the new count. Since 2026-04 adjusting an item that is not stocked at the location succeeds and creates an inactive level, so a success does not prove the stock is sellable.