In Chapter 15, the man in the yellow suit makes a deal with Winnie's family: he will bring Winnie back to them in exchange for their land. So, what the man in the yellow suit proposes is a trade: Winnie for the land (that contains the magic spring, of course).
According to the man in the yellow suit, he and the constable will bring Winnie back. The man in the yellow suit then sets out with the constable, who seems dubious about his strangely-dressed companion.
In Chapter 18, the man in the yellow suit goes to the Tuck household, where Winnie is staying. The Tuck family are at breakfast, but the man in the yellow suit interrupts their meal with an announcement: he is there to take Winnie back to her family. The Tucks are suspicious, but Angus tells everyone to let the man speak.
In Chapter 19, the man in the yellow suit reveals that he knows about the spring that bequeaths eternal longevity to anyone who drinks from it. He announces to the Tucks that the woods and spring now belong to him. Additionally, he is determined to sell the water to anyone who is interested. The man in the yellow suit proposes that the Tucks join him in his new business venture.
So, the man in the yellow suit first makes a deal with Winnie's family: the woods (and spring) for Winnie's safe return. Then, he makes a deal with the Tucks to sell and advertise the "benefits" of the magic spring. All the Tucks have to do is to show him where the spring is and then act as his assistants during "demonstrations."