Small Industries Development Bank of India (SIDBI), the leading financial institution for the MSME sector, has invited banks and NBFCs to test its invoice-based financing app GST Sahay, said a source aware of the developments.Currently, SIDBI is testing the GST Sahay platform with its own members and also on the RBI Sandbox, a controlled environment for testing financial products. Thirteen transactions with its customers have already taken place on the app.