Mr. Foster is essentially being blackmailed by the man in the yellow suit. The Fosters have owned the Treegap wood for a very long time. The constable says,
"They're the first family around here, you know. Proud as peacocks, all of 'em. Family-proud, and land-proud, too."
The constable, knowing the value Mr. Foster places in his land, is shocked when the man in the yellow suit tells him that Mr. Foster has agreed to sell a portion of the land to him.
Mr. Foster really has no choice in the matter. His granddaughter Winnie has disappeared, and the man in the yellow suit has shown up at his house, claiming to know what has happened to her. The man has made it clear that he will not share his information, however, unless Mr. Foster agrees to sell him the woods on his property. The man in the yellow suit sets forth the conditions of his cooperation clearly, stating,
"...it looks as if I'm the only person in the whole world who knows where to find (Winnie)...I've got what you want, and you've got what I want...I want the wood and you want the child. It's a trade. A simple, clear-cut trade" (Chapters 15-16).