what can i do with inconsistent billing balancese

A medical bill was sent to a collection agency and I agreed to pay a monthly amount. The account was taken from the collection agency by the original billing co.The balance before it was taken back was $285.00 then was billed $424.00 and put under investigation.The next bill after investigation $303.00 when speaking with the rep. she noted that there was an additional $51.56 which was not on the bill. Next to the amount, sates current patient responsibility. Now they are to send me a different bill with a balance of the $303.0 + $51.56 which will amount to $354.81. I requested to print on the bill to come, final payment for i keep receiving inconsistent amounts.