Question

Is it permitted to make a tattoo dot (almost invisible) for the purpose of marking?
I saw that making a tattoo on a slave is forbidden, but one is not liable, since it is done in order to mark him so that he will not run away.
The question is whether there is room to be lenient here, since it is a tiny dot that is almost invisible; it is not a letter nor even a shape.
(This is a mark meant to know where the injection site is; there is no medical need for it.)