Depends on your state laws, actually. In most states, Accord and Satisfaction (aka Restrictive endorsements) do NOT work. In others, they will work but ONLY if you follow specific steps.
"Some collection agencies will agree to settle with you for far less than you owe and then turn around and hire another collection agency to collect the difference. However, in many states this is illegal. Once a creditor deposits or cashes a full payment check, even if she strikes out the words payment in full or writes "I don't agree" on the check, she can't come after you for the balance. The states in which this law is enforced:
Arkansas
Kansas
Louisiana
Maine
Nebraska
North Carolina
Pennsylvania
Utah
Virginia
Wyoming Colorado
Connecticut
Georgia
Michigan
New Jersey
Oregon
Texas
Vermont
Washington "
Looks like you may be one if the lucky ones. Do some more reading here:
http://www.carreonandassociates.com/4settle.htm