A peer-to-peer complementary currency can be a powerful tool for promoting exchanges that make use of underutilized computing resources in a trusted way. i-WAT is proposed such currency based on the WAT System, a polycentric complementary currency using WAT tickets as its media of exchange: participants spontaneously issue and circulate the tickets as needed, whose values are backed up by chains of trust. i-WAT implements the tickets electronically by exchanging messages signed in OpenPGP [J. Callas et al., (1998)]. This paper proposes an extension to the design of i-WAT to facilitate trades. In particular, we propose additional "multiplication" tickets whose values increase over time. By deferring redemption of such tickets, the participants can receive the premium realized by increased debts of the issuers. A game-theoretical analysis shows that this feature is incentive-compatible: the issuers have no reason to strategically default despite the increased debts. A reference implementation of i-WAT has been developed in the form of a plug-in for an XMPP instant messaging client. We have been putting the currency system with the proposed feature into practical use.