The Hoops sold the Irish striker to Bulgarian club CSKA Sofia in August last year but claim that they are still owed £275,000.
Celtic are now at war with Sofia and have made a formal protest to FIFA as they strive to get the cash owed to them.
Sheridan left Celtic Park after four years in the first team setup. Along the way he enjoyed loan deals at Motherwell, Plymouth Argyle and St Johnstone before moving to Bulgaria last summer. He has also represented Republic of Ireland on three occasions.