If “cash is king,” then Middle East coffers are irresistibly enticing. During a recent tour of Saudi Arabia and the Gulf states, Deputy Treasury Secretary Robert Kimmitt applauded the “growing role” of Arab banks in the U.S. economy. Treasury is seeking buyers for its newly acquired bailout assets because more than $1 trillion in cash is urgently needed to rescue the largest U.S. banks. However, cash from the Arabian Gulf comes with a vital string attached: Islamic banking, erroneously viewed as an ancient practice. In fact, Islamic banking is a newly invented institution: “Neither classical nor medieval Islamic civilization featured...