From page 439 The Life of Mark Twain - The Middle Years 1871-1891:

December 30 Tuesday – Through his attorney in Boston, George L. Huntress, Sam filed a “bill in equity” (complaint) against Estes and Lauriat booksellers of Boston for advertising Huckleberry Finn at the price of $2.25, below the $2.75 subscription rate [N.Y. Times, Dec. 31, 1884, p3; MTBus 318]. (See Jan. 14, 1885 entry.)