Dear Hal:
Leo Marx defined "down in the bills" on page 29 of the 1967
Bobbs-Merrill edition of Huckleberry Finn (Indianapolis, New York) as
"written down in the specifications (bills as in the phrase, fill the
bill); thus, foreordained." The standard slang and regional English
dictionaries do not list the expression. Your student could look at
the Glossary and Reference List in the Mark Twain Library or Mark
Twain Works editions of Huckleberry Finn (University of California
Press, 2001 or 2003) for a list of standard resources. Like Marx, the
glossary defines it as "Written down in the specifications, hence
predestined, foreordained."

Victor Fischer
Mark Twain Project.