It depends upon whether it's for stimulus # 1 or stimulus #2. The first stimulus check had provisions for garnishment for back child support. The second payment protected people from garnishment completely.
I am assuming you are referring to the Recovery Rebate Credit that is calculated on your 2020 income tax return. If so, you will need to reflect that you did in fact receive the payment even though it was used to offset your past due child support.
You would not be eligible for the Recovery Rebate Credit. If you claimed it on your 2020 income tax return, the IRS will review the amounts entered and will adjust your tax return accordingly.
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