Accepted a postdoc offer by just emailing, have not signed any contract, what to do if other offer comes
Your honor is at stake here. The email should constitute an agreement, even if not legally binding yet. You can, of course, by mutual agreement with the PI, still change your acceptance, indicating that it is tentative because of the restriction. This should be easy and natural, if the condition was raised after your "acceptance." 

Just don't get yourself in the position of the PI thinking he has "caught" you and you later "get away." The consequences will be on you.