Upon the allowance of a credit or refund of any tax erroneously or illegally assessed or collected, or of any penalty collected without authority, or of any sum which was excessive or in any manner wrongfully collected, interest shall be allowed and paid on the amount of such credit or refund at the rate provided in section 63-3045, Idaho Code, from the date such tax, penalty, or sum was paid or from the date the return was required to be filed, whichever date is the later, to the date of the allowance of the refund, or in the case of a credit, to the due date of the amount against which the credit is taken; provided, however, that in case of a voluntary and unrequested payment in excess of actual tax liability, no interest shall be allowed when such excess is refunded or credited.