Now that Mr. Boffin is gone, Silas Wegg and Mr. Venus sit up to catch their breath. They've been wrestling for a couple of minutes now, and it's time for Wegg to tell Venus what he found in the dust mound behind the Boffin house.
Wegg leads Venus out to the dust mounds, where he digs up an old hat box. Inside the box is a piece of paper. It's a version of Old Man Harmon's will that leaves all of his money to the government and not to Mr. Boffin. If Wegg were ever to show this will to the authorities, Boffin would lose everything.
If you know your 19th-century literature, you know what this means… it's blackmail time! Woo-hoo! And Wegg plans on splitting whatever he gets out of Boffin with Mr. Venus.
Mr. Venus suggests that they retreat back to his store to look at the paper some more. Once they get there, though, he traps Wegg and threatens him until Wegg gives him the will. Venus won't take the chance of letting Wegg double-cross him.
With that all settled, Wegg and Venus decide to wait until Mr. Boffin has sold off all his dust mounds so they can cut into the money that Boffin will make from the sale.