為替 is a money order or exchange rate. How did they pick these two kanji? From what I can tell, it's not kunyomi, onyomi, ateji, or gikun.
From http://gogen-allguide.com/ka/kawase.html this is the relevant part:
漢字の「為替」は当て字だが、「かえる(替える)」と「する(為る)」で、文字通り「替えることを行う」意味である。
So apparently is is just ateji that happens to have meaning that matches the word.
To add to ssb's explanation, what is usually called ateji are actually two phenomena: 当て字 (音): to use kanji only for their sound, eg 矢鱈 義訓 (訓): to use kanji for their meaning, and assign a fitting word to the kanji (compound), eg 為替
It's often not necessary to distinguish between these two methods (of writing words), but it's something to keep in mind.
Also, 為替 is the 連用形 of 交わす.
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日本国語大辞典(精選版) かわせ【為替・交】 (動詞「かわす(交)」「かわす(為替)」の連用形の名詞化) (1) 取り交わすこと。 (2)(中世は「かわし」という。) ... dic.yahoo.co.jp/… – blutorange Jul 16 '13 at 14:30
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