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This paper aims to study the way in which an economy must choose its exchange rate policy, in the case of external unbalance which in turn may affect external debt sustainability. A macro-dynamic external growth model can be presented when real income and international capital flows exert a fundamental influence. The main result is that the dynamics of external net claims are non linear and very sensitive to the economic context in which external debts are contracted. For endebted countries, external sustainability needs appropriate solutions that will vary depending on their specific characteristics. In particular being able to apprehend any policy based on a rise or fall in the exchange rate will by definition include the variables imposed by foreign trade elasticities.Classification JEL: E40; F21; F31; F44.