@IntoTheMirror the immediate solution to your problem is to effectively create an “Accounts Payable (Suppliers)” subsidiary ledger journal by entering a correcting Purchase invoice as per this:
Which produces this:
Report looks like this:
If you wish to claw back the overpayment of 3.68 to the supplier change the top line amount to 17.50
If the original purchase was a cash purchase create a “sundry” supplier account.
@Hennie putting your EOY WIP journal through the Accounts Payable subsidiary ledger, in this way, I suspect will have the desired effect that you require.
In terms of a permanent solution I support this suggestion by @lubos as it will allow flexibility without having to make a choice as to purchase or sale which would allow too much possibility for error.