With my clients, we'll initiate a dispute with the credit reporting agency. At the same time we will request a complete accounting in writing of all payments received. Usually, it will take a manager to authorize the accounting. With these things combined we can usually get to the bottom of the problem.
You have rights, and remedies, under both the Fair Debt Collection Practices Act and under the Fair Credit Reporting Act. You need to get a copy of your credit reports, and a copy of the documents relating to the subject account, and meet with an attorney. You need an attorney in order to properly demand that the account be corrected or to "set up" the collection agency and/or original creditor for suit.
I don't think there is a legal issue with the debt collection agency reporting the account on your credit within 30 days. You have legal rights to dispute and correct the debt amount on your credit report. Further, you have rights to request that the credit reporting bureau validate the debt.