Section 1692f of the FDCPA addresses your concern. A debt collector may not use unfair or unconscionable means to collect a debt. A collection agency violates the the FDCPA if the debt collector collects any amount (including any interest, fee, charge, or expense) unless such amount is expressly authorized by the agreement creating the debt or permitted by law.
You should send the collection a letter, certified mail return receipt, with proof of the agreement to pay $2400 and proof of the overpayment. The collection agency should correct the problem. I have good working relationships with most collection agencies. Therefore, if you cannot solve the problem on your own, please feel free to contact me.
Information on Avvo should not be construed as legal advice, as each case is different. For information about your specific case, please contact a consumer law attorney, or contact me at www.agrusslawfirm.com. The FDCPA has a fee-shift provision. This means, the collection agency is responsible for paying your attorney's fees and costs.
Hire a local lawyer familiar with consumer debt and collection law. A letter from a lawyer, citing both the federal and Maryland versions of the fair debt collection practices laws, together with supporting documentation proving the over payment, should be sufficient. If not, both the federal and Maryland statutes provide for substantial civil penalties and an award of attorney's fees in addition to any out-of-pocket loss you may have suffered.