You may be able to get a Montgomery GI Bill refund if you meet the requirements listed here. 

One of these must be true:

• You entered active duty after June 30, 1985, or 
• You served a combination of at least 2 years of active-duty service and 4 years of Selected Reserve service after June 30, 1985

And all of these must be true:

• You paid the $1,200 MGIB buy-in when you joined the military, and
• You chose to use your Post-9/11 GI Bill benefits instead of your MGIB benefits, and
• You had unused MGIB benefits when you started using Post-9/11 GI Bill benefits instead, and
• You’ve used all of your Post-9/11 GI Bill benefits, and
• You were receiving a Post-9/11 GI Bill monthly housing allowance on the day your entitlement ended