I am not familiar specifically with the 'karma spell' you speak of, I've honestly never heard of it before.
But now, lets move on to the answer itself.
If you choose to believe in karma, you should know what you are dealing with before you spread around misconceptions and jump to conclusions about karma. One should know what karma is before making a thread about it and asking about it questions which might be answered incorrectly. It seems like people say karma like a bell ring, as it meant nothing and as it was just a meaningless concept or game to play around with.
Karma, to say it shortly without too many details, explanations or descriptions, is what we call an anonymous divine force.
When it comes to anonymous divine forces, just like karma or what you speak of, it's not a good idea to endeavor to manipulate it. This probably isn't the best idea you could have. It's not something you should mess around with as it was a child's play or any toy you play around with, innocently.
You should read more and know what karma is before attempting to mess up with it or attempting to manipulate it.
Just take care of yourself and take responsibility over your actions.