This paper proposes a barter currency system called i-WAT to promote sustainable economy in peer-to-peer internetworking. i-WAT itself is a peer-to-peer system, -without necessitating a central point of authority. It uses a digitally signed, electronic form of promissory note as the medium of exchange. This paper illustrates how it should assist internetworking of peer-to-peer systems, and discusses, in particular, how trust can be maintained in such a currency system. Among other means, the current design of i-WAT allows the notes to be transported over Jabber[8, 9] instant messaging protocol. A prototype of an i-WAT checkbook has been developed as a plug-in for a Jabber client. We are beginning to experiment on the actual usage of the currency system using the checkbook as well as provisional web applications.