  We study a novel pricing operator for complete, local martingale models. The
new pricing operator guarantees put-call parity to hold for model prices and
the value of a forward contract to match the buy-and-hold strategy, even if the
underlying follows strict local martingale dynamics. More precisely, we discuss
a change of num\'eraire (change of currency) technique when the underlying is
only a local martingale modelling for example an exchange rate. The new pricing
operator assigns prices to contingent claims according to the minimal cost for
superreplication strategies that succeed with probability one for both
currencies as num\'eraire. Within this context, we interpret the lack of the
martingale property of an exchange-rate as a reflection of the possibility that
the num\'eraire currency may devalue completely against the asset currency
(hyperinflation).
