  The purpose is to compare the perfect Stochastic Return (SR) model like
Islamic banks to the Fixed Return (FR) model as in conventional banks by
measuring up their impacts at the macroeconomic level. We prove that if the
optimal choice of investor share in SR model {\alpha}* realizes the
indifference of the financial institution toward SR and FR models, there exists
{\alpha} less than {\alpha}* such that the banks strictly prefers the SR model.
Also, there exists {\alpha}, {\gamma} and {\lambda} verifying the conditions of
{\alpha}-sharing such that each party in economy can be better under the SR
model and the economic welfare could be improved in a Pareto-efficient way.
