<human>: Summarize the given text in a few sentences.

My husband learned he had cancer about two years ago and we've been dealing with medical bills ever since. We've paid every single valid bill (after thoroughly reviewing). Thankfully i have a good job and money isn't a problem. 

Today we received a notice from a debt collector for a $20 bill that we never knew about for some procedure my husband had done back in late 2014. I called the service provider (not the debt collector) and found that they had the wrong address on file. They told me that all of the bills they sent were returned due to the bad address. 

After verifying with my insurance company (against my EOB) i determined the debt was valid and that I did actually owe this company the $20. I called back and offered to pay the original debt and was told they didn't have access to the account anymore because it had been sent to the collection agency. They said the only way to pay the debt was to do it through the collection agency. 

So now i'm not sure what to do. I've always been told not to deal with debt collectors. The two times I've ever dealt with collection agencies my credit report still got hosed and i was stuck with a negative item on there for years. I feel like we are screwed no matter what we do. If we pay the debt collector they can still tack a negative item on my husband's report. If i don't pay them, they'll stick it on the report too. 

What should i do?
<bot>: Received a letter from collection agency for a valid bill that was never sent to us due to incorrect address. Not sure how to pay without screwing up our credit report.