@Ballentine 

Yes, technically the security deposit is a liability but at this point that’s irrelevant because you’re refunding the security deposit and you want to debit the account that was credited by the security deposit line item on the invoice.  You do that with a refund receipt using the same security deposit line item that was used on the invoice.  You don’t want to make any entry to A/R as this has nothing to with A/R.