Section 6521 may be applied in the correction of a certain type of error involving both the tax on self-employment income under section 1401 and the employee tax under section 3101 if the correction of the error as to one tax is, on the date the correction is authorized, prevented in whole or in part by the operation of any law or rule of law other than section 7122, relating to compromises. Examples of such law are sections 6212(c), 6401(a), 6501, 6511, 6512(a), 6514, 6532, 6901 (c), (d) and (e), 7121, and 7459(e).

If the liability for either tax with respect to which the error was made has been compromised under section 7122, the provisions of section 6521 limiting the correction with respect to the other tax do not apply.

Section 6521 is not applicable if, on the date of the authorization, correction of the effect of the error is permissible as to both taxes without recourse to such section.

If, because an amount of wages, as defined in section 3121(a), is erroneously treated as self-employment income, as defined in section 1402(b), or an amount of self-employment income is erroneously treated as wages, it is necessary in correcting the error to assess the correct tax and give a credit or refund for the amount of the tax erroneously paid, and if either, but not both, of such adjustments is prevented by any law or rule of law (other than section 7122), the amount of the assessment, or the amount of the credit or refund, authorized shall reflect the adjustment which would be made in respect of the other tax (either the tax on self-employment income under section 1401 or the employee tax under section 3101) but for the operation of such law or rule of law. For example, assume that during 1955 A paid $10 as tax on an amount erroneously treated as “wages”, when such amount was actually self-employment income, and that credit or refund of the $10 is not barred. A should have paid a self-employment tax of $15 on the amount. If the assessment of the correct tax, that is, $15, is barred by the statute of limitations, no credit or refund of the $10 shall be made without offsetting against such $10 the $15, assessment of which is barred. Thus, no credit or refund in respect of the $10 can be made.

As another example, assume that during 1955 a taxpayer reports wages of $4,200 and net earnings from self-employment of $900. By reason of the limitations of section 1402(b) he shows no self-employment income. Assume further that by reason of a final decision by the Tax Court of the United States, further adjustments to the taxpayer's income tax liability are barred. The question of the amount of his wages, as defined in section 3121, was not in issue in the Tax Court litigation, but it is subsequently determined (within the period of limitations applicable under the Federal Insurance Contributions Act) that $700 of the $4,200 reported as wages was not for employment as defined in section 3121(b). Therefore, the taxpayer is entitled to the allowance of a refund of the $14 tax paid on such remuneration under section 3101. The reduction of his wages from $4,200 to $3,500 would result in the determination of $700 self-employment income, the tax on which is $21 for the year. Under section 6521, the overpayment of $14 would be offset by the barred deficiency of $21, thus eliminating the refund otherwise allowable. If the facts were changed so that the taxpayer erroneously paid tax on self-employment income of $700, having been taxed on only $3,500 as wages, and within the period of limitations applicable under the Federal Insurance Contributions Act, it is determined that his wages were $4,200, the tax of $14 under section 3101, otherwise collectible, would be eliminated by offsetting under section 6521 the barred overpayment of $21. The balance of the barred overpayment, $7, cannot be credited or refunded.

Another illustration of the operation of section 6521 is the case of a taxpayer who, for 1955, is erroneously taxed on $2,500 as wages, the tax on which is $50, and who reports no self-employment income. After the period of limitations has run on the refund of the tax under the Federal Insurance Contributions Act, it is determined that the amount treated as wages should have been reported as net earnings from self-employment. The taxpayer's self-employment income would then be $2,500 and the tax thereon would be $75. Assume that the period of limitations applicable to subtitle A of the Code has not expired, and that a notice of deficiency may properly be issued. Under section 6521, the amount of the deficiency of $75 must be reduced by the barred overpayment of $50.