61 STAT.] 80TH CONG., 1 ST SESS.-CH. 258 -JULY 16, 1947 (1) within three years after the assessment of the tax or (2) prior to the expiration of any period for collection agreed upon in writing by the Assessor and the taxpayer before the expiration of such three- year period. The period so agreed upon may be extended by sub- sequent agreements in writing made before the expiration of the period previously agreed upon. SEC. 11. REFUNDS.- Except as to any deficiency taxes assessed under the provisions of section 5 of this title, where there has been an over- payment of any tax imposed by this article, the amount of such over- payment shall be credited against any income tax or installment thereof, whether such tax was assessed as a deficiency or otherwise, then due from the taxpayer, and the balance shall be refunded to the taxpayer. No such credit or refund shall be allowed after three years from the time the tax was paid unless before the expiration of such period a claim therefor is filed by the taxpayer, and no tax or part thereof which the Assessor may determine to have been an overpay- ment shall be refunded after the period prescribed therefor in the Act appropriating the funds from which such refund would otherwise be made. The amount of such credit or refund shall not exceed the portion of the tax paid during the three years immediately preceding the filing of the claim, or if no claim was filed, then during the three years immediately preceding the allowance of such credit or refund. Every claim for credit or refund must be in writing, under oath- must state the specific grounds upon which the claim is founded, and must be filed with the Assessor: Provided, That if it shall be determined by the Assessor, the Board of Tax Appeals for the District of Columbia, or any court that any part of any tax which was assessed as a deficiency under the provisions of section 5 of this title was an overpayment, interest shall be allowed and paid upon such overpayment at the rate of 4 per centum per annum from the date such overpayments were paid until the date of refund. SEC. 12. CLOSING AGREEMENTS.- The Assessor is authorized to enter into a written agreement with any person relating to the liability of such person (or of the person or estate for whom he acts) in respect of any income tax for any period ending prior to the date of the agree- ment. If such agreement is approved by the Commissioners within such time as may be stated in such agreement, or later agreed to, such agreement shall be final and conclusive and except upon a showing of fraud or malfeasance, or misrepresentation of a material fact-the case shall not be reopened as to the matters agreed upon or the agree- ment modified; and in any suit or proceeding relating to the tax lia- bility of the taxpayer such agreement shall not be annulled, modified, set aside, or disregarded. SEC. 13. COMPROMISES.-(a) AUTHORITY TO MAKE. --Whenever in the opinion of the Commissioners there shall arise with respect of any tax imposed under this article any doubt as to the liability of the tax- payer or the collectibility of the tax for any reason whatsoever, the Commissioners may compromise such tax. (b) CONCEALMENT OF ASSETS.-Any person who, in connection with any compromise under this section or offer of such compromise or in connection with any closing agreement under this title or offer to enter into any such agreement, willfully (1) conceals from any officer or employee of the District of Columbia any property belonging to the estate of the taxpayer or other person liable with respect of the tax, or (2) receives, destroys, mutilates, or falsifies any book, docu- ment, or record or makes under oath any false statement relating to the estate or the financial condition of the taxpayer or to the person liable in respect of the tax, shall, upon conviction thereof, be fined not more than $5,000 or imprisoned for not more than one year, or both. 355 Crediting of over- payment, etc. Ante, p. 352 . Time limitation. Limitation on amount of credit or refund. Filing of claim. Interest on over- payment. Ante, p. 352 . Penalty.