Where the transaction involved constitutes a sale of standing timber under this chapter and the selling price is stated in such conveyance as being determinable in the future on the basis of footage removed or on a stumpage basis, it shall be the duty of the seller to execute and file with the County Treasurer the foregoing affidavit, stating, in addition to the other requirements, the legal description of the real property on which such standing timber is located and an estimate, to the best of his knowledge, of the selling price ultimately to be received, and shall pay to the County Treasurer under this chapter a sum equal to one percent of such estimated selling price, which sum shall not be credited by the Treasurer to the school fund but shall be retained in a separate account. On the expiration date of such timber contract, if not extended, or at the time cutting and removal is completed, whichever is earlier, it shall be the duty of the seller to execute and file with the County Treasurer an additional affidavit in the above form setting forth the price actually paid. In the event such amount results in a tax greater than the sum theretofore paid on the estimate, the seller shall pay such additional amount to the County Treasurer who shall thereupon place this amount, together with the sum originally deposited, to the credit of the school fund in the usual manner. In the event such amount be less than the original estimate, the Treasurer is hereby directed to refund the excess payment to the taxpayer and credit the balance to the school fund as above.