International economics is an essential part of economics and of a great importance in all economic systems, including the economic system of Islam. Not so many studies, however, have been done about this issue from the Islamic viewpoint. In this paper, investigating the economic relations between Muslim countries, we examine their balance of foreign payments and extract their balance of payments (BP) curve. Our hypothesis here is that "eliminating interest rate would not lead to recession in the Muslims' international transactions, and the expected turnover rate can be an efficient substitute for interest rate." On the basis of this hypothesis, and using a library approach, exploiting the ideas of Islamic economics experts, and utilizing the mathematical relations, we have found that economic relations between the countries making an Islamic system are definable, that eliminating the interest rate from Muslims' transactions does not have a negative effect on them, and that such substitutes as the expected turnover rate could be proper substitutes for interest rate. With some assumptions, the relations between Islamic and non-Islamic countries are arguable, with which we have not dealt in this paper.