Soy Sauce (しょうゆ, Shouyu) is an item in the first Suikoden. It can be obtained in Kirov as the penultimate item in the fetch quest there and can be traded for Soap in order to recruit Sarah.
Suikoden
In order for Tir to recruit Sarah in Kirov, he had to get her some Soap as per her request. This was only doable by completing a series of trades in order to obtain it. Successfully trading Salt for the soy sauce allowed Tir to complete the mission when he traded the soy sauce for the elusive soap.