To take a deduction for a SSA repayment on your 1040:
1. Go to Deductions & Credits
2. Go to Other Deductions and Credits
3. Go to Other Deductible Expenses.
If it's less than $3,000, click Yes on Less Common Expenses and list it as a repayment.
If it's more than $3,000, click Yes on Any Other Deductions?. You'll put it by Claim of Right Repayment (Only If Over $3,000).