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You can cancel an unconfirmed transaction any time without anything happening. You get hit with the 7-day market lock if you cancel a confirmed and "live" trade. As long as you see the "awaiting confirmation" message, you are good.

Unconfirmed trade offer can be canceled anytime (in your trade offer page it say "Awaiting confirmation" or something).
If you cancel trade offer when it's already confirmed (it say something like "Trade offer confirmed, on hold X days"), both person will get 7 days trade hold