A supplier payment can be made without any previous documents or journal entries needing to be created first.

It will be an AP debit until it is reconciled with something.

If you create the Supplier invoice after the payment, register another payment of $0 and apply the debit to the AP credit generated by the invoice.

EDIT (to show workflow):

1) Accounting --> Suppliers --> Supplier Payment

2) Click Create

3) Fill in the information for Supplier and Total:

4) Click Validate

You will see that OpenERP detects that there is a difference amount equal to the Total and will set the Payment Difference to Keep Open.

5) To verify, Accounting --> Suppliers, open the Supplier record and examine the Total Payable amount on the Accounting tab.