The clubs include Atletico Madrid, Sporting Lisbon and Malaga but no British sides.
The sanctions follow the first of Uefa's financial fair play rules coming into force regarding unpaid debts. The prize money will be withheld pending further investigation.
The action was taken by the Uefa Club Financial Control Body (CFCB) investigatory chamber, and Uefa said: "The clubs participating in Uefa club competitions had to provide information regarding the status of any overdue payables as at June 30 2012.
"The CFCB investigatory chamber has identified that important overdue payables towards other clubs, and/or towards employees or social/tax authorities existed in 23 cases."