Primary historic Salary Data is provided by Doug Pappas and SABR.
In late 2012, we added the work of Dr. Michael Haupert. Dr. Haupert is a professor of economics at UW-LaCrosse and he has painstakingly gone through the Hall of Fame's records to find player contract information for nearly 7,000 player seasons primarily from 1915 to 1969.
Generally, there were be one salary per season and we will note any disputed numbers. Note that under one circumstance, there WILL be multiple salaries for the same player in the same season. It happens when the player was released during a multi-year contract, but signed by someone else -- the new team pays him the major-league minimum, with the other club responsible for the balance. Both will be listed. Similarly, you'll run across a few salaries for seasons after a player has apparently retired -- those are the players released during multi-year contracts who weren't picked up by anyone else.
For some players, there is no salary information, official or unofficial, available for players who weren't on the 25-man roster or the DL as of either Opening Day or August 31. The media reports use these cutoff dates for their published salary lists, and the official list -- "Joint Exhibit 1" for use in salary arbitrations -- looks only at the August 31 roster. Anyone called up after September 1, or for a few weeks in midseason, slips through the cracks. It's reasonable to assume that all rookies who fit this description are earning the minimum.
Contract data comes primarily from the Joint Exhibit One, Paul Riker, media sources, and Cot's Contracts. See Cot's for extremely detailed player contract history.
Players who are traded are listed with their full season salaries and not the salaries paid to them by each team. Any payroll information will reflect this difference.