Why didn't the stranger want Mr. foster to come along with him when he went to get Winnie?

The stranger didn't want Mr. Foster to accompany him because he likely believed that Mr. Foster would be a hindrance or could complicate the situation. The stranger may have wanted to avoid drawing attention to their actions or thought that Mr. Foster's presence could jeopardize their plans. Additionally, he might have felt it was safer or more efficient to go alone to retrieve Winnie.