The offer was valued at roughly $300 million over 10 years with no opt-outs, according to Janes.
However, Harper instead elected to explore the market, rejecting what would have been the largest contract in free-agent history. Giancarlo Stanton signed a 13-year, $325-million contract with the Miami Marlins in 2014, but that was an extension rather than during free agency.
Nationals general manager Mike Rizzo confirmed the team tried to sign Harper before he hit the open market.
"(We) took advantage of our exclusivity late in the season," Rizzo said, "(but) couldn't reach a deal."
The Nationals extended a qualifying offer to the former National League MVP last week, so if Harper does sign with a new team, Washington will receive a supplemental draft pick in 2019 as compensation.