Service providers make money through providing services, not currency arbitrage- so it's usually good business practice to structure longer-term contracts to allow price adjustments for relevant currency fluctuations. I say "usually" because there are plenty of situations where you are perfectly fine to take on the currency risk on a contract.
In those situations, hedge your risk by marking up the relevant underlying exchange rates, and don't include the underlying rates in the contract if no adjustments apply- otherwise your customer may look at it someday (when it's in their favor) and demand an adjustment that puts you in a pickle if your contract is otherwise silent on currency exchange fluctuation.
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