Dispute partial charge

I’d like to dispute a charge on my latest bill. Long story short, I got a call from Xfinity at the end of June telling me my bill will go up since my original contract is about to end. After negotiation with the representative, I agreed to switch to a new plan but only after my original one year contract ends in August, which means I don’t expect to see any extra charges for the service I got before August. Now the new bill came in and guess what, I see a partial charge which seems to be the price difference between 2 plans (a charge for the period from the day they called me to the end of my original contract). I called customer service, the representative didn’t seem to know what the charge is for and kept telling me this is consented when I signed the new contract. But I asked Xifinity specifically not to end my previous plan in advance and they assured me there wouldn’t any extra/unexpected charge for the original service I got. And that price difference itself is not correct either (didn’t count in my autopay discount correctly). I’d like the partial charge removed from the new bill. Anyone here can help with it?