RE: Use the Inventory button to remove the marking on the order line before you update any financial dimension.
This happens because your Warehouse dimension is financially tracked.
If you want to change the warehouse on the purchase order you have to:
1 - remove the marking (in your purchase order lines--> Inventory --> Marking)
2 - Change the warehouse in your purchase order
3 - Approve or confirm your purchase order (so the sales order can see that stock)
4 - Change the warehouse in your sales order.
5 - Create the marking again in your sales order. (in you sales order lines --> Inventory --> Marking)
Its not the fastest way but since your warehouse dimension is financially tracked, its the only one.
Best Regards
