Audi A6

I took my Audi A6 to them in July 2018 for a oil leak on the gearbox, (under their warranty) after 8 weeks I got it back from them but then the gears start slipping when the gearbox is hot, I took it back to them on the 6th of November 2018 and I am still waiting to get it back from them, every time I phone there is a new problem and they never come back with any feedback. Now after 7 month they want me to pay R15 000 to fix there mistakes. Please do not take your car to them