Minn. Stat. § 297I.75

If before the expiration of the time prescribed in sections 297I.65 and 297I.70 for the assessment of tax or surcharge or the filing of a claim for refund, the commissioner and the taxpayer have consented in writing to the assessment or filing of a claim for refund after that time, the tax or surcharge may be assessed at any time before the expiration of the agreed-upon period and a claim for refund may be paid at any time before the expiration of the agreed-upon period plus six months. The period may be extended by later agreements in writing before the expiration of the period previously agreed upon.