Yeah, commonly referred to as "the fine print", "the catch", "the hitch",
etc.

The point is someone came forward and Trudeau paid. If nobody had come
forward to prove Bush's service he wouldn't have paid, at least not before
the election. He'd have been trumpeting "still no witness" all the way to
election day! He paid very quickly, and very quietly, after Lt. Col.
Calhoun verified Bush's service. If he hadn't the Republicans would have
lambasted him for reneging on his offer, and he'd probably be facing a
lawsuit.

Then why is the offer still posted?

http://www.doonesbury.com/strip/bush_guard.html

Apparently Calhoun's dates did not jibe with the dates that Bush claims to have
been in Alabama, and the money was never paid.
Maybe he mistook The Kingfish for GWB.