Unhappy buyer .What to do ???

As a professional diff builder I would say that he needs a second opinion because the terms his diff guy is full of nonsense. The turning torque of a gear set is due to bearing preload. The gears really have nothing to do with turning torque. This diff guy is trying relieve the buyer of more money because he is either incompetent or dishonest. There are two victims here and the buyer needs to find an honest person to assess his diff. Two things are very important one is backlash and the other is gear contact pattern. Gear break in will not affect turning torque. I hope this sheds some light on your situation. Both the buyer and seller are probably being scammed.