What is Tanpo? Basically it means security collateral. In this story, the Suzumiya family have run deep in dept that they pawn off their three daughters Yukari, Ai, and Midori as guarantees for the money they'll receive. Well, they mistake some piece of trash called Ishihara Yuujirou for the Daikoudan Group. Unfortunately, Ai-chan is the one that the rest of the family/staff coerce into tempting Yuujirou with her body to initiate sex with her in order to secure money for the debts. (Source: MU) Included one-shot: Volume 6: Oniichan to Yobaretakute