Soap is an important washing tool used for removing dirt and other residues from objects and clothing. When McDohl first met Sarah in Kirov she agreed to join the Toran Liberation Army under one condition: That he procure her some soap. To do this, the hero had to embark on a trading spree within the city, purchasing Sugar in the item shop and exchanging that for a Yardstick, the yardstick for Salt, the salt for Soy Sauce and, finally, the soy sauce for soap.
Once the soap was brought to Sarah, it turned out she had some extra and so didn't take the piece the hero had traded for.
- Genso Suikoden Kiwami Encyclopedia, page 67