Xfinity agent lied about changing my bill

I contacted a chat agents because I had a new promotion applied to my account and they promised my bill would be adjusted to $58 accordingly but I was still charged $78 today. Then I start a new chat and they claim they can’t fix it and will apply credit to my next month’s bill. How am I supposed to trust them to actually do that when they already said my bill would be lowed and it was not. I have screenshots and everything