Workshop wants money despite VW warranty

Hello. I have been reading silently for a long time, but today I need some help. It is about a Golf 7 which is still in the warranty. In May the workshop A changed the clutch cylinder. After about 2 months and 3000 kilometers the gears could only be inserted difficult. Since driving was not possible so, workshop B, which was just around the corner, would be commissioned. Workshop B carried out adjustment work and ventilation of the system. After 4 weeks the order with VW cancels the payment, because Workshop A is not supposed to have worked properly. The workshop contradicts this. Of course, the 2nd workshop insists on the payment. VW refers to the autonomy of its contractual partners and does not want to help. What possibility there is not to settle the invoice. At no time has Workshop B announced that any costs could arise for me. Thank you for the help