What does Pap want Huck to do?
Pap wanted to get the money that Judge Thatcher was supervising for Huck. At the beginning of the story, most of it was invested but Pap knew that Huck was allowed to get one dollar every day from the Judge. Huck decides he doesn't want the principal (the original six thousand dollars) or the interest after he has a run of bad luck indications, so he makes a deal with the Judge.
You want to sell all your property to me-not give it. That's the correct idea." Then he wrote something on a paper and read it over, and says: 'There-you see it says 'for a consideration.' That means I have boght it of you and paid you for it. Here's a dollar for youl Now, you sign it." So I signed it, and left.
Pap wasn't aware of this transaction when he first reappeared and demanded that Huck go and get his money. Pap investigated and had to give up on getting all the money at once, but was able to physically threaten Huck to force Huck to obtain periodic loans from the Judge of lesser amounts of money, all of which went to pay for liquor for Pap.