Upon the sale of county property which the county purchased with funds from the county road fund, the proceeds of the sale shall be paid into the county road fund. If, in addition to county road funds, other funds were used by the county to purchase the property, then the amount to be paid into the county road fund shall be a portion of the proceeds determined by using the ratio of the amount of county road funds used by the county in purchasing the property to the full purchase price paid by the county.