Is it permissible to pay Zakat on money in the form of dates, by estimating the value of the dates at their selling price and deducting that from the obligatory Zakat on money, knowing that the zakat payer has owned the dates for three months?

The dates you bought for sale are considered trade goods. Therefore, you must appraise them at market price and then add their value to your cash or other trade goods. If the total reaches the nisab (definition) (minimum threshold) and a hawl (one lunar year) passes, then zakat (definition) is obligatory on the entire amount, and it should be paid in cash. It is not permissible to pay zakat in dates or any other trade goods, especially if paying them as goods would not be more beneficial for the poor.