Fraud at Handy repair service center.

Hello.
I came to mobile phone service kiosk and asked to change a protective glass on camera, it was cracked.
This type of glass cost around 3-4 Euro on Ebay, and not a big deal to change. The worker pretended not to speak english (my german is not sufficient to discuss details of repair. After "repair" he turned out to speak fluent english) and convinced me to pay 100 Euro in advance because he has to "order this part", but having asked why so expensive, he replied "we don't do hobby repair, we replace a whole module" (Kamera). I have a receipt with price of "Installationsdient" 20€ and "Kamera Iphone 8+ Black" 79.90€.
In fact, when the part "arrived" and was replaced, there was a glass glued on both cameras, and this was presented as a repair for 100€. (Glass is never to be glued on camera, because objective lenses are moving in order to focus and stabilize image).
After long discussions that this is not a proper way to repair phone, and in fact the camera was not replaced, i was said that "what you read in reciept does not matter, in our system "Kamera Iphone 8+ Black" means a glass".
This is not how it should be, and moreover, the worker did not provide a copy of a contract that i signed when payed. Having asked one afterwards, he said i can "f*** off, we dont have it, go to police, to lawyer or wherever, we dont care".
But i do! My questions regarding this situation:
--Is there a law that obliges a company to provide me a copy of the contract?
--Is this contradiction between what payed (Kamera for 79.90€) and what was done (glass for 4€) considered as Betrug according to german law (in fact it definetelly is).
My plan is to find a company that can make a Gutachten of improper repair, make a complain at Verbrauchzentrale and find a lawyer that will not say "Hey, 100€ does not wort it, just go with it".
You may reply in German as well, i use google translator.
Vielen Dank.