Is the increase resulting from currency conversion rightfully the exhibitor's, or is it permissible to take it?

The apparent view—and Allah knows best—is that this increase belongs either to the proxy or to the buyer. If the buyer paid the amount in Omani currency as the price for the car, then the increase belongs to the showroom. However, if the amount was paid on the basis that you would receive the equivalent of the car's price in Dirhams, then the increase is the property of the buyer, and it remains a deposit with him unless he gifts it to you or to the showroom.