It's convenient that telepathy can pass through the absolute barrier that doesn't even let light through.
Well... Technically he is the master of the barrier so he can leave the barrier? So i think it just apply the same thing to attacks and skills he uses
This
may be true. It isn't just telepathy but
his magical energy that goes through the barrier (I recall that he has sent his magic to the slimes when they were within it). We would have to see if he is physically stopped by the barrier by him acting upon it physically (I don't think it was this manga where the MC used a "similar" barrier as something to stand on, but could always be wrong.)
If course the danger of having "exceptions" to something called "absolute" is that it becomes no longer "absolute"... If mana can be hijacked then so can the barrier.
It seemed like the dispel just went past it, rather than expending any energy trying to do anything at all to the liquid. That's a different reaction.
I had another response to this (based on how various magics usually work), but decided to check before I made an idiot of myself. Glad I did.
The author uses "dispell" in an unusual way. Typically "dispell" is similar to barrier (but instead of being hollow it's a solid) that acts to remove, counter, deactivate or disrupt the "contained" effect. It very rarely is a "missile" type magic that is "released" and moves to/through it's target, like was shown here.
Anyway, since I went back and checked, I can say with certainty that he didn't mention "rather than expending any energy trying to do anything at all to the liquid" but "The dispel magic just went past it. There is no change in the black liquid. It wasn't a curse?"
The distinction is important because of what you are quoting. It's not that he
could tell there was no effect because he could feel that the magic wasn't being "used up" (all indication for this universe is that the only time they "use" magic is to "pay" the "cost" of the magic... unless it was stated elsewhere that they can "reinforce" the magic, but I - with my non-existent memory - don't remember that; I'm happy to be proven wrong). Ergo it's entirety possible that there
was an effect, but it was so minimal that he couldn't tell/detect it; think of a massive amount of water being dumped on a small fire, sure, "some" water will evaporate, but it might not be visible. Or the reverse, a small amount of water on to a massive fire, sure, "some" fire may be extinguished, but it may be impossible to tell.
And, of course, the light novel may have explained it differently than the manga showed it (i.e. exactly how you said it), but I haven't read the book, only the manga, so I can only go based on what was shown here.