The director of revenue shall deposit promptly all funds received by him in payment of the tax imposed by sections 148.610 to 148.710 to the credit of the “Savings Association Tax Fund” or “Credit Union Tax Fund” which funds are hereby created. He shall maintain such funds in institutions, savings associations and credit unions selected by him and approved by the governor, state treasurer and state auditor. Thereafter he shall, until the time set for the distribution of the net proceeds of the tax, invest all moneys within the fund in the same manner as state funds not needed for the immediate expenses of the state are invested; provided, that in all instances in which the state treasurer may use or invest in banking institutions, the director of revenue may use or invest in credit unions or in associations. All interest earned upon the moneys so invested shall be deposited in the respective credit union or savings associations tax fund and all such interest shall be returned to the various county treasurers within thirty days of tax distribution. Each county shall receive that percentage of the total interest earned as its share of the tax paid bears to the total amount of the tax received by the director of revenue. A statement of the exact amount of interest due each political subdivision in such county determined by the pro rata share of the proceeds of the tax received by such political subdivision bears to the proceeds of the tax received by the county shall accompany each payment.