The federal lawsuit says the league missed the initial deadline for repayment in October and another on Dec. 1.
The suit, filed Monday, names as defendants league co-founder William Hambrecht and his revocable trust, which guaranteed the loan.
Calls to the league's legal and public relations offices by The Associated Press Thursday weren't immediately returned.
Cuban declined additional comment when contacted by the AP.
Cuban's loan coincided with an announcement by the league last April that he would be an investor.
The five-team league completed its second season in November when the Las Vegas Locomotives defeated the Florida Tuskers for the championship.