If the supplier and the customer are the same. This means you are buying from part x and also selling to party x. And you want to see the complete account together.
Credit Sale to Supplier: You can create a customer and link it with the supplier ledger account.
Also Allow Credit Sale = Yes. This is how you can make a credit sale to the supplier.
Credit Purchase from Supplier: Currently the list is coming from Sundry Creditor + Cash Group + Sundry Debtor .