Very difficult is this peculiar intricate way in which karma works. There is no such thing as karma sitting outside on a tree. It is not a thing whose existence we can visualise somewhere. Just as we consider diseases to be a peculiar maladjustment of the physical functioning of the body, rather than a thing that is sitting outside the body and existing separately there, so also the karma is not sitting outside, waiting to harass us.
Karma is the peculiar automatic reaction set up by the cosmic forces in proportion to the action performed by an individual. The reaction will be exactly in proportion to the action that we perform. In a way it, looks like tit for tat, In a crude way, we may say it is like that.