Man, if you are willing to send the guy money for the guiness you owe him, then it must be a good enough reason to stop being a cheap *** (sorry to swear, i mean it in a friendly context).
If you buy guiness in a bar/pub, which is the normal place you buy someone a beer when you owe them a beer, it's about ¥1,000 per beer (im rounding up, but guiness is expensive).
It would be much nicer though, to buy it for him the next time you see the guy. Because not only do you buy the person you owe a beer a beer in a pub, but you drink one with him too!