Do I have to return painted cabinets even though client disputes finish and infers non payment?

We have a signed contract for painting kitchen cabinets specifying procedure,type of paint and prep work. We also have an email confirming color and the treatment of the inside of doors.Specifically we state that we plan on cleaning, sanding and color matching what's there and seal with wax. They respond with approval and we procede accordingly. On re-installation day wife see doors closed and open revealing original wood finish. She comments how great they look but husband calls hours later to say he thought backs would be painted. We never said backs would be painted.In fact email confirms the treatment planned. We tried to show good faith by offering $100 off final payment but he insists he wants doors painted. We still have some cabinets. Can we hold on to them til there's resolution?