The Matter Ledger is a running list of all A/R related transactions related to a single matter. From the Matter record, select Matter Actions, then Matter Ledger. This will open a side-panel on the right of the record. Any Payment, Credit or Invoice will show in this view, totaling to the Matter’s current A/R balance (which can be corroborated by the A/R report in Matter view).
Clicking in the Type column will open the record of the line item selected (i.e. Clicking on the word Invoice in the first line will open that invoice).