The note that you signed when you applied for the student loan contains all the terms and it cannot be changed after the loan is approved and funded. Only the interest rate may vary, depending on the terms of your particular loan, if it is a variable rate loan.
The BBB is not a good place to complain about this problem. ACS may also have an ombudsman or other dispute resolution office. You also should send a written dispute to the new company to advise them in writing that you dispute this new arrangement. Please see the sample letters on my web site, such as letter 2.1, which is for billing disputes regarding credit cards, but you can modify it to suit your needs. You will want to send it by certified mail, return receipt, and retain a copy for your records. You should include a photocopy of the original note and make it clear in your letter that you intend to continue paying the $150 payments, per the note.
Also, you should continue paying the what you can. If you cannot afford $400, until this problem is resolved, pay the $150 as you did before every month, so you have make all your required payments on time, per your original contract terms. Be sure to retain proof of payments, such as keep a copy of the cancelled checks in a file or on your computer, so you have evidence of making all the payments to the new company if you need it.
If they continue to mess up the accounting of your loan payments and credit history, perhaps then you will need to consult directly with a consumer attorney. and possible litigation.
Robert Stempler (please see DISCLAIMER below)
NOTICE: The above statements are provided for general information purposes only and are not intended as legal advice or advice of any sort for a specific case or legal matter. If you do not have a signed attorney-client fee agreement with the Consumer Law Office of Robert Stempler, APLC ("the Firm"), then until such written fee agreement is provided and signed by both a prospective client and attorney for a particular case, neither Mr. Stempler nor the Firm will represent you nor will they be your attorney in any matter and you remain responsible for retaining your own attorney and for compliance with any and all deadlines and for any statutes of limitations that may pertain to potential claims. Comments made on a public forum, such as Avvo.com, to not have any confidentiality because others may read them. If you desire a private consultation with Mr. Stempler that is confidential, please go to www.StopCollectionLawsuits.com and submit a free eCase Review.
You're asking what your contract rights are, and no one on Avvo knows what the exact language in your particular contract is.
You need to review your loan docs with Wells Fargo to see if they can assign your loan to someone else, like ACS. If they can, then you have to see if the terms of your loan can change, and if so how and when.
PLEASE READ THIS BEFORE YOU COMMENT, EMAIL ME OR PHONE ME. I'm only licensed in CA. This answer doesn't make me your lawyer, and neither do follow-up comments and/or emails and/or phone calls, and you shouldn't expect me to respond to your further questions if you haven't hired me. We need an actual agreement confirmed in writing before any attorney-client relationship is formed. This answer doesn't constitute legal advice, and shouldn't be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue.