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.
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.
From http://gogen-allguide.com/ka/kawase.html this is the relevant part:
So apparently is is just ateji that happens to have meaning that matches the word.