In Chapter 16, Huck is feeling guilty and ashamed of his role in helping Jim escape. Although he doesn't want Jim to be hurt, he also still feels connected to society and its expectations for slaves, and Jim's talk about stealing his wife and children away makes Huck feel sick. Unable to listen anymore, and feeling incredibly guilty, Huck resolves to turn Jim in without telling him; he sets off the raft in a canoe, and immediately comes across two men with guns in a skiff. Huck weakens and lies, telling them there is a white man on board.
"I reckon we'll go and see for ourselves."
"I wish you would," says I, "because it's pap that's there, and maybe you'd help me tow the raft ashore where the light is. He's sick -- and so is mam and Mary Ann."
"Oh, the devil! we're in a hurry, boy. But I s'pose we've got to. Come, buckle to your paddle, and let's get along."
(Twain, The Adventures of Huckleberry Finn, gutenberg.org)
Knowing that the reveal of Jim will mean his capture and that Huck might be returned to his father's care, Huck continues to embellish the lie, allowing the men to think that the people on the raft have smallpox. Unwilling to risk infection, the men move on, and even give Huck some money to buy medicine in the next town. Huck feels even worse about this lie, since it caused him to gain money, but he also feels relieved that he helped Jim escape capture again.