Incorrectly sent to collections

Hello I really need someone’s help. My account was recently closed in August 2022 and while waiting for a pending credit I was sent to collections. I was told I was only responsible for $50 after returning my equipment and to wait for a new invoice to pay it. I called I several times but kept getting the run around. I logged into the chat but it timed out several times waiting for a live agent. I updated my new address but I never received the new invoice in the mail. I just paid the $50 and but sure enough today it showed as a ding on my credit report right before I was going or make an offer on a condo. I have worked very hard on my credit the past 7 years and it was flawless and now it has all come crashing down. I already paid the remaining $160 balance through the xfinity app but I need to get the collection removed from credit report in order to save my new mortgage. I spent all day getting transferred around on the phone and I can’t seem to get anyone that understands my issue and I always get transferred and then disconnected. Who can I speak to about this issue? I really need your help I was given some bad information and not I’m in a really bad spot. Please help me.