The official NAF to USD exchange rate is fixed at 1.79 to 1, but most gas stations do the exchange in their favor at 1.75 to 1. If I get gas on the Dutch side, I ask what the station's exchange is prior to fueling and have never had a problem.

In the OP's case, the gas is in his car and the cash is in his wallet. I would have told the attendant the most I'm paying is $51 and explain why (89 guilders at 1.75 exchange is $50.86). Let the attendant call his manager or the police if he wants. Letting him get away with taking advantage of tourists is just giving him the boldness to keep doing it.