A useful feature in TM2 invoicing is the ability to deal with excess payments. The scenario is common, when an insurance company pays a claim for a patient, but leaves the excess. You have the problem of then billing the excess to the patient.
In TM2 it's easy, just create the invoice for the Insurance Company for the full amount as usual. When the payment comes in, allocate it to the invoice, leaving the excess amount unpaid. Now click on Transfer remaining to Patient and the system will automatically bill the remaining amount to the patient and clear the Insurance Company's account. Just print off an invoice for the patient and send it out.