she did trick him when she sealed the life debt deal. she forced him into the deal, and deceived him, as she knew he didn't know anything. she didn't give him the chance to make a proper deal and negotiate. sure, she thinks she won a bit in that deal, but it also undervalued her own life, indirectly, insulting herself
![Big grin :D :D](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
the templars warned MC that the Elves would try to trick him out of the deal and make him ask for more. if they didn't warn him beforehand, the way Medb phrased it, it could be seen as a real deception, with malicious intent (as it would make the deal null and void), it would make it seem like MC made the request to change the deal if he fell for it.
also, she tries to trick him again multiple times by asking if the deal is now done even though she knows the requirements are not met.
it was not a playful cultural thing at all. it was a battle in itself. even if she didn't try her hardest to win because she has a soft spot for humans and MC in particular, she did try a few moves for her side to come out on top. the life debt is supposed to be an honorable ancient tradition, and the fact that the Elves still tried to deceive MC with it is malicious and ill intent. at least, that's how I see it