The Skiller can decide whether he wants to give up to 5 minutes free. Its good in order to check if they match. And in case that the 5 minutes free was enough for the whole service the user can tip the Skiller and everyone will be happy

How do you prevent people from abusing this 5 minute grace period?
We don't. its up to the Skiller to give them. If the Skiller notice that the user is not serious he can end the call immediately.
I see. Is the call charged per second or?
It's probably paid per second yeah.
I hope there will be option for some fixed amount too,so Skillers dont drag the call just to get more money.
In case they do it the user can open a dispute. We courage them to finish the call quickly by adding a "Tip" option.
So the Tip would be just a little something extra for the Skiller? Would the Tip have some sort of limit - just asking because some Skiller might take advantage of that.