There are four options in this case:
First: You pay the Ring and you travel for sure where you wanna go.
Second: You say "no" while he is asking for the Ring and then you have to pay 50 gp, but there is a chance for not landing on the right spot.
Third: You dont have a Dwarven Ring with you. Then he will only ask for the 50 gp, but there is also a chance for not landing on the right spot.
Fourth: You wear a Dwarven Ring. Then he will also only ask for 50 gp (same like the third option).