The source of Mr. Heron's money is unknown, or at least it is not made clear in the book.  It is true that William Heron was a surveyor, and as such, could have made "a lot of money", since, as Tim's father noted, "surveyors always knew about the good deals on land and could get rich speculating".  Still, Mr. Heron's apparent wealth must have far exceeded what might be expected for a simple surveyor. 

Tim says about Mr. Heron that the gentleman had

"...been to Trinity College in Dublin, and...had been elected to the General Assembly in Hartford, but he'd been pushed out of it by the Patriots for being a Tory...he was rich, too, although nobody knew where he got his money from...he owned a black man and he had other servants besides".

Beyond that, and the fact that he had dealings with the infamous Benedict Arnold, little more information is given about Mr. Heron's background (Chapter 5).

In the last section of the book, entitled "How Much of this Book is True?", the author states,

"William Heron was a real but somewhat mysterious figure in history.  It appears that he was working for the Americans as well as the British.  At the least he must have been a double agent, but historians are not sure exactly what role he played in the war".

It would appear that William Heron's background, including the circumstances of his financial standing, are as clouded in muncertainty as are the nature of his true loyalties during the Revolutionary War.