The New York Times reported Friday that the union recently sent a letter to player agents informing them that Upper Deck "had not paid a large number of players money it owed them".
The Times indicated that the email was sent by Evan Kaplan, who runs the Association's trading cards and collectibles division and was sent to all player agents. A recipient of the email provided a copy of it to the Times.
The paper reported that the union indicated it was working with Upper Deck on a plan to make sure that the players were paid. The email, however, "strongly advised" players not to return signed cards or enter into any new deals with Upper Deck until the payments were made by the company.
Upper Deck was recently forced to pay Major League Baseball an undisclosed amount of money after the two sides settled out of court over Upper Deck's 2010 baseball products that MLB said violated trademark laws.
Upper Deck lost its baseball trading card license at the end of last season.
The company also recently settled a lawsuit with Konami Entertainment over its creation of non-approved Yu-Gi-Oh! trading cards by agreeing to pay Konami an unknown amount of cash.