Sardex is an electronic B2B mutual credit system that has been operating on the island of Sardinia since 2009 as a complementary currency. In contrast to other funding mechanisms, it allows private funding driving social and environmental impact to be endogenously generated within a given geographically limited socio-economic context, rather than injected from exogenous sources. By drawing on different strands of monetary theory, sociology and anthropology, we argue that mutual credit is central to a form of social finance and social impact investment that we identify with sustainable development, i.e. stable and constructive integration of market activity with democratic institutions and socio-cultural values and structures. The empirical basis of the paper consists of approximately thirty semi-structured in-depth interviews of Sardex circuit members and founders over 4 years. We conclude that Sardex as a collective social enterprise is best understood through an interdisciplinary perspective that demonstrates its sustainability through the different levels of its workings and its strengths as a hybrid multilayered system.