Midland Credit Managerment - Incorrect Past Due Balance

I have an item on my credit report from Midland that shows an incorrect past due amount. This debt was 'sold' to Phillips & Cohen in DE and I contacted this company and settled the account and received a letter from Phillips on behalf of their client Midland Credit Managment that the account was satisfied.
I've filed disputes with the major credit agencies and they stated it as 'correct'. What would you suggest as the next step? I was thinking of calling Midland to ask for a letter stating it was satisfied that I could then provide to the credit reporting agencies.

Thanks in advance for your help,
Alan