Is there anyway to stop leaks in a underground return line for an inground pool?

Consult an underground leak detection service that specializes in pool leaks. They may be able to fix with out tearing up the deck.

Or, you can grab a kit. There are kits nowadays that you can use to find and stop leaks. Also, you can use a dye test! What you do is get a dye syringe and the dye will actually lead you to the leak by "leaking" into the hole. Then you can fix it with vinyl putty! And, VIOLA, it's fixed!!!!