Dealer principal swears, come back on workmanship

I brought a motorcycle to Ryder Motorrad on the first week of September, the technician had work done to the engine and when I picked the bike up. It only lasted 10 minutes. After weeks and weeks of asking what is the status, I get asked if I'm going to pay for part direct or through BMW. They gave me a courtesy bike and told me they wont charge me, only to get a bill close to R7000. Now they wont release the bike to me, and they dont want to cover it under workshop warranty after only 10 minutes of taking it home. Apparently the shims were done and now there is a bent valve as per technician. The first talk I had with the dealer principal, was with swearing and talking to me which I didn't appreciate. They are falsifying documents stating I signed for courtesy bike when I did not sign anything when I took the courtesy bike, not to mention a verbal agreement that no charge shall apply because of bike in the workshop. So the question is, what do you warranty when you do shim on a bike? The only reason you would do shims is if valve clearances are not in order, so your directly affecting valves when adjusting shims. I still do not have my bike in my possession.