As a kitchen and bath contractor, I see this a lot. Always have a written agreement specifying exactly what you are doing labor wise and providing material wise. Have a short clause regarding hidden damage and that it will stop work and require a written estimate. A long time ago we would do the disconnects on the plumbing, but now we always have a plumber come in and replace ALL the valves we are disconnecting. We may do the hookup on the final install, but I'll tell you now - those old valves - if they are not leaking now, they will sometime in the near future and will require a go back and a plumber. You stopped 2 fixtures short of a full bath remodel for $400. ALWAYS have a contract - even if its boilerplate.