Minn. Stat. § 375.171: CREDIT CARDS.

A county board may authorize the use of a credit card by any county officer or employee otherwise authorized to make a purchase on behalf of the county. If a county officer or employee makes a purchase by credit card that is not approved by the county board, the officer or employee is personally liable for the amount of the purchase. A purchase by credit card must otherwise comply with all statutes, rules, or county policy applicable to county purchases.