They aren't allowed to make unauthorized withdrawals, They most likely made the $600 withdraw under auspice that you had previously given permission for them to process ACH transactions.
To protect your self, whether dealing with SAM
(a personal vendetta of mine by the way) or any collection agency NEVER give give them a check by phone credit card by phone or any authorization to do regular ACH withdrawals. In order to do this you have given them all the information they need to go in and take money out of your account at anytime. Also, never do Western Union or Money gram, they charge excessive fees (up to $15.00) and your payment won't be paid any faster. Always insist on payment only by Money order, Cashiers Check, or certified checks only (even your own paper check they can get the necessary information to take withdrawals).It would also be highly recommended to send these payments CMRRR or PMDC (priority Mail w/ certificate of mailing and delivery confirmation, they can not refuse to accept mail sent in this manner like they can CMRRR).
Using Money Order, Cashiers or Certified check may cost you some extra also (not more then $5.00 depending on which you choose to use, but can be as little at $.50 )you are provided with a receipt, and can check with the issuer to see that the instrument was cashed.
It is very simple. They Must accept any payment you send them, and they must post it to your account (this does not meant they agree to any payment arrangements however). Even if they do not agree to your proposed payment plan, send them the money (assuming you acknowledge the debt or it has been validated). By doing so you are showing you are making a reasonable effort to address the situation within your means. Should they press the issue and take it to court, you will have evidence to show the court you ave been trying to work something out, but they are refusing to cooperate.