You should make the refund immediately

Technically you should refund the deposit immediately when the tenant moved out. However, the law do recognize that it takes time to estimate the cost and processing the document, so the law gave you 21 days so that you will have plenty of time to process the refund.  And you want to take advantage of this and wait till the last day? This is just rediculous.
Like other said, you should tell your customer why the refund can not be issued immediately and how long they need to wait. You should give them a date that they can expect to receive the refund check.  This is Business 101. And you are intentionally ignoring the customer? This is just unbelievable.