The net realizable value of the accounts receivable is the price at which the transaction is expected to be settled. If you have sold goods and you have invoiced the other party for $1,000 the net realizable value is broadly speaking $1,000. There are exceptions where for example the other party is in financial difficulties and you know that the amount that you will receive is less than what originally recognized. In this situation, the net realizable value will be less and you need to impair your asset to bring it to the value at which the transaction is expected to be settled at.