Barnes, a retired Newark police officer, knew the signature alone could fetch a goodly sum. For example, the Raab Collection in Ardmore currently lists four documents with Hancock's signature for $22,000 to $75,000.
But Barnes appreciated his letter's greater value as a piece of state history.
He took it to the Delaware Public Archives in Dover and gave it to public archivist Stephen M. Marz.
In making the donation, Barnes said, "as to where the letter came from, I cannot say, but it is now where it belongs."
Among the archives' more than 10.4 million documents, it's the only one signed by Hancock.
"We are so excited to have this," Marz said. "It's wonderful to have a John Hancock document that relates directly to Delaware history."
At the time he wrote the letter, Marz said, Hancock was president of the Continental Congress. Yet he closed his letter to state officials, "Your most Obedient Servant."
Marz said the letter was now in better shape than when it was found. "We sent it to the Conservation Center for Art and Historic Artifacts in Philadelphia," he said.
There, the surface was cleaned and adhesive tape removed, said Tom Summers, the archives' manager of outreach services. Chemical baths added an alkaline preservation buffer, and tears were mended with wheat starch paste and mulberry paper, he said. The process cost $2,005.
The letter is considered priceless, but its financial worth will remain unknown, Marz said. "We don't put a value on anything," he said.
But adding to its importance is its crucial timing in the Revolutionary War.
"It was between the Battles of Trenton and Princeton," Summers said, so Washington had crossed the Delaware River once and was about to go again.
Americans won both battles, he said, but Delaware suffered a key loss in the one after Hancock's letter. Col. John Haslet of Milford, first commander of the famed First Delaware Regiment, died Jan. 3, 1777, in the Battle of Princeton.
Hancock's letter soon will be digitized and posted on the archives' website, he said.