How does William dorrit come into his fortune in little dorrit?

The book is somewhat vague on this account, but William Dorrit is said to be the heir of a substantial fortune that had remained maturing, unclaimed, until Arthur Clennam - aided by Mr. Pancks, Young John Chivery, and Mr. Rugg - endeavored to look into William Dorrit's history. Mr. Pancks' dogged character is mainly responsible for the discovery, as he invested his own capital and took out a loan to fund the venture.

While not explicitly said, the reader must assume that the fortune was due to a relative's passing, and essentially, the fortune was just waiting to be found, and Mr. Pancks' ability to work through bureaucracy allowed it to be found.