Before revision:  Under mild assumptions, it is shown that a foreign cash flow's worth in local currency is identical to the value of the cash flow in local currency for which the Radon-Nikodym derivative with respect to the foreign cash flow is the forward FX rate . While the derived valuation principles are ubiquitously used in theory and practice, they are usually neither stated in this general way, nor do derivations of them exhibit the here presented mathematical rigor, which is based on the definition of a market as an equivalence relation of exchangeability .
After revision:  Under mild assumptions, it is shown that a foreign cash flow's worth in local currency is identical to the value of the cash flow in local currency for which the Radon-Nikodym derivative with respect to the foreign cash flow is the forward FX rate .