The system does not allow you to apply a RAP to multiple Matters of a Client.  Instead, you will need to move the Retainer balance from the Matter on which it is sitting to the Matter you are needing to pay off in Orion.  To move retainer money from one Matter to another, you will use the Retainer Transfer program by opening the AR Manager, and choosing ACTION, Retainer/Trust Transfer.  The following screen will appear:

Choose the Retainer option and enter the Client-Matter Number on which the retainer balance currently exists.  Enter the amount you want to transfer. Click the ADD TARGET Button and enter the Client-Matter to which you want to move the retainer money.  You may distribute the money to more than one Matter.  Although not recommended, you may move all or a portion of the retainer balance to a Matter under a different Client.  Once you have fully disbursed the amount, click the PERFORM TRANSFER Button.  Orion will create a RTO transaction on the Client-Matter from which the balance was moved and a RTI on the Client-Matter(s) to which the balance was moved.  Once you have established the balance on the correct Client-Matter, you may then create a RAP on that Client-Matter and move forward.