Could you explain the error, rather than just say that it is a common error? How can I agree to pay in a situation which happens only if I was predicted to disagree?
(I don't object to having precommited to agree to pay if Omega makes his request; that would indeed be a correct decision. But then, of course, Omega doesn't appear. The given formulation
Omega asks you to pay him $100. Do you pay?
implies that Omega really appears, which logically excludes the variant of paying. Maybe it is only a matter of formulation.)
This problem is roughly isomorphic to the branch of Transparent Newcomb (version 1, version 2) where box B is empty, but it's simpler.
Here's a diagram: