This is a supplement to salary which helps the employees pay the cost of converting currencies in foreign countries.
It would not properly be called a "bonus", but rather an "allowance".

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Note added at 15 mins (2012-08-10 03:12:08 GMT)
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Allowance:
A sum of money paid (regularly) to a person, typically to meet specified needs or expenses.
More info »Dictionary.com - Answers.com - Merriam-Webster - The Free Dictionary

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Note added at 19 hrs (2012-08-10 22:50:04 GMT)
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Revised answer: allowance for certain expenses while abroad, paid in the respective foreign currencies.
(This is based on reading Polangmar's cited reference.)