[Slick]

Do the right thing. Inform them and pay what you agreed. You would want the same kind of treatment. You will feel very good about yourself for being honest. Too often in this world we resort to "legal technicalities" to take advantage of the situation whenever possible. Legal technicalities arise when people find new and sneakier ways of not doing the right thing. We really learned (or should have learned) life's most important rules before kindergarten...

do the right thing,
play nicely with others,
share,
help others that are less fortunate,
if it isn't yours, don't take it,
keep your hands to yourself,
if you make a mess, clean it up,
etc.