Should I send a validation letter?

I had a recent medical bill go to a collection agency. I contacted the provider right away and they agreed to accept payment in full within 30 days and stated they would pull it back from the collection agency. I paid the bill during the same call but would like to try to keep this off of my credit report. If I send a validation letter to the collection agency, will they be able to validate a settled debt? I'm hoping that they will simply not respond.

Thanks for the input.