The causal relationship between the rates of return on deposits of Islamic banks and deposits of conventional banks in an economy with dual banking systems is a significant area of study in the literature. While studying this causal relationship, vast majority of the existing studies consider domestic currency deposits. In this paper instead of domestic currency deposits, I consider foreign exchange currency deposits. By employing Toda-Yamamoto causality analysis, a causality relationship for all the maturity groups is found, but not for the 12-months maturity. For dollar accounts, the causality relationship between rate of returns of Islamic and Conventional banking is found to be two-way. For Euro accounts, this causality relationship is found to be one way, where the interest rate of conventional banks is Granger cause of profit share of Islamic banks.