that suuucks. a trade ought to be a trade if that's what you agreed to. not to worry, in my experience, karma can be selectively very bitchy.
i agree with The Joker -- gifting makes karma not a bitch. i've been able to save my a$$ on a number of occasions by calling in favours. not that i'd call it that, but when you've helped people out karmically and they're decent enough people to be aware of that, you can almost always count on them coming through for you in your next time of need.
i helped a couple out once, nice people i met through a friend, i'd provided some small mothers but what they really appreciated was that while i was there i gave them cloning pointers that doubled their success rate. fortunately, i haven't yet had the need to hit them up, but *years* later whenever i occasionally run into them they still tell me to let them know if i need anything - genetics, babies, whatever.
also having had favours called in unexpectedly/inconveniently from people i knew i owed one to, i've come to believe that if you can keep dealing with people you know you have similar values with, trades usually work out in the long run, even if it ends up being "A gives B badly needed genetics" for "B helps A with midnight move" -- you know?
and those people who screw you over in your quest for similarly-valued people? they'll never know the privilege of having *you owe *them a favour, and they make those few-and-far-between kindred spirits that much more valuable.