Your relationship to stock is strictly based on the Order 1 fields. Placing this field with the other order items is simply placing a copy of the same field on the other locations and thus you have the same price for all. You'd need separate relationships to different table occurrences of Stock inorder for your current layout to work.
I suggest you modify your design so that this complexity is no longer necessary. Typical Invoicing systems use this table structure:
Invoices---<LineItems>----Stock (called products in many examples)
Then you use a portal to LineItems on your Invoices layout to record each item sold on that invoice.
Here's a demo file created by another forum participant and uploaded to a different FileMaker forum you might want to take a look at: