The NFLPA sent out an email to player agents on Wednesday asking them to report any improper dealings with NFL teams.
The email, obtained by ESPNDallas, tells agents, "We have heard reports of a concern that teams are working in concert to 'peg,' 'rig' or 'set' market prices on player contracts. If you believe or have information that the teams have been colluding during this free-agency period, you have a responsibility as an agent of the NFLPA to come forward and share that information with us."
The email also states the union has "heard anecdotally" that some NFL teams are inaccurately reporting they have salary-cap issues in regards to signing veteran players.
NFL spokesperson Greg Aiello said of the email: "Player signings in 2013 have been characterized by robust spending and intense competition. Anyone seeing collusion in this market is seeing ghosts."
There are five NFL teams with more than $20 million in salary-cap space, led by the Jacksonville Jaguars, who have $28.3 million in cap space. Based on NFLPA figures, the average cap space for NFL teams as of Wednesday is $9.6 million.
League and union officials didn't respond seeking comment regarding the email. The two-day NFLPA meetings take place in Las Vegas on Thursday.