We have this situation occur frquently.
 
Our solution was to create a checking account called "Prepaid Goods Not Received"
 
We normally wire payment to our vendors that require prepayment, so we use the Transfer Bank Account Funds key, but you could simply create a AP voucher against the prepaid goods not received account.
 
Then, when we receive the goods, we invoice them using AP-C but make it a COD rather than an invoice, and select the prepaid goods not received as the bank account.
 
Finally, the accounts can be reconciled to 0.  The advantage to this approach is that you never lose track of a payment (which we had done in the past).

It works great!  Thanks so much for your help!!