If, on the other hand, you still expect that you can encounter some other Omega-like thing which will present you with such a scenario, doesn't this make the deal repeatable, which is not how the question was formulated?

In a repeatable deal your action influences the conditions in the next rounds. Even if you defect in this round, you may still cooperate in the next rounds, Omegas aren't looking back at how you decided in the past, and don't punish you by not offering the deals. Your success in the following rounds (from your current point of view) depends on whether you manage to precommit to the future encounters, not on what you do now.