26 USC 6401 - Amounts treated as overpayments

(a) Assessment and collection after limitation period. 
The term overpayment includes that part of the amount of the payment of any internal revenue tax which is assessed or collected after the expiration of the period of limitation properly applicable thereto.
(b) Excessive credits 

(1) In general 
If the amount allowable as credits under subpart C of part IV of subchapter A of chapter 1 (relating to refundable credits) exceeds the tax imposed by subtitle A (reduced by the credits allowable under subparts A, B, D, G, and H of such part IV), the amount of such excess shall be considered an overpayment.
(2) Special rule for credit under section 33 
For purposes of paragraph (1), any credit allowed under section 33 (relating to withholding of tax on nonresident aliens and on foreign corporations) for any taxable year shall be treated as a credit allowable under subpart C of part IV of subchapter A of chapter 1 only if an election under subsection (g) or (h) of section 6013 is in effect for such taxable year. The preceding sentence shall not apply to any credit so allowed by reason of section 1446.

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(c) Rule where no tax liability 
An amount paid as tax shall not be considered not to constitute an overpayment solely by reason of the fact that there was no tax liability in respect of which such amount was paid.