Not a very streamlined process but here is what you have to do.
 
1.  Presumably you already have a Credit on the AP Aging for this vendor and this is what is being reimbursed.  Use AP-B and enter a Voucher for the same amount.  Post the GL to any account you want but remember where you posted it.  This will Credit AP and Debit the offset account.
2.  Use AP-F/G/H and pick the credit and voucher to pay and generate a dummy $0 check to clear the aging.  No GL impact from this step.
3.  Use GL-B and enter a Cash Receipt (Type 2) Journal, debiting Cash and crediting the account used for the offset in step 1.  This will offset the posting from step 1 (which is why it made no difference what account you used) and debit Cash and make a Deposit entry to the Check Register.  Final net GL posting is to Debit Cash and Credit AP.