Double entry on bad debt reserve?

Bad debt reserve is an amount set aside by companies in the event that some creditors would not be able to pay their debt. A double entry is to write off the bad debt from the accounting books. For example, company A sold goods to company B for $100 on credit. This is entered as $100 debit in company A's ledger. Later on, company B is unable to fulfill payment on the goods. Company A therefore must write this off in their ledger to keep their assets in check. This will now be entered as $100 credit in the ledger.