Mitsubishi gearbox

Bough a gearbox from Japan Auto. The casing was broken, and i was told that i could replace it with my original one. When gearbox mechanic opened the gearbox to replace the casing, he noticed that the gearbox inside was in bad shape. Gears were worn bad. Gearbox has done way over 200 000 km. Japan Auto is known to only have engines and gearboxes with low milage only. I explained to them that gearbox was in bad shape. They refuse to refund me. They say that i had opened the gearbox. How am i to change the casing, if i dont open it?