First of all, the person claiming that this is legal must be a collector, or just a plain idiot!
Now, NEVER speak to ANY debt collector on the phone!! ALWAYS DEMAND EVERYTHING BE IN WRITING!! A paper trail is absolutely essential when you end up in court.
You have the LEGAL RIGHT to dispute any collection attempt, and/or any garnishment.
They CANNOT legally get a garnishment order without first suing you and winning. Were you ever sued on this? If not, that is your defense to the garnishment order. They are a 3rd party collector, regardless of who they are collecting for.
If indeed Sally Mae "sold" the loan, you are only dealing with a junk debt buyer who paid about 2 cents on the dollar for your old debt!
NEVER SIGN AWAY YOUR RIGHTS!!
You need to get an attorney TODAY!!!
GET AN ATTORNEY ASAP!!!!!