I'm developing a web application that will be used by medical clinics. Clients get an invoice for the treatment(s) they got. However, sometimes a part of the costs will be paid by a third party, like a coupon company (if the client has bought a coupon for a treatment) or a health insurance company. How should I call these companies?
Clients are called 'clients' in the system. I've thought about calling the companies 'debtors', but clients can be debtors as well. Does someone know a word that would fit this meaning?