How to refuse a request politely

My boss is a real estate broker. Recently he received a letter from the lawyer requesting a copy of the agreement of purchase and sale of a property five years ago. They have provided the client's name and the property address. However we couldn't locate such information from all our records. My broker called the lawyer and left a voicemail telling him that there's no such deal. The lawyer sent another letter to my broker urging for the provision of the information request. He even mentioned that he saw his signature on the agreement. Due to the litigation process, he has to obtain the copy directly from our office.

My broker could only recall a similar transaction with another broker and also requested for the other broker to help. Eventually, nothing could be find after a detail search from both parties. I was asked to write a reply to the lawyer.

Please help to make my letter looks proper and polite.

Dear Mr. Anderson,

I refer to your request on March 14, 2006 for a copy of the agreement of purchase and sale of 16 Mandel Crescent. I have replied to you on March 18 that there was no such trade record in my company for this property.

However your letter on March 19 indicated that you've seen my signature with regard to this transaction. Upon your request, my company performed a detail search on all our archive records. I even contacted my co-operating broker at ABC Realty requesting them to search for the document. The result was still nothing could be find for that property address. I regret to inform you that despite of all the efforts we have spent on the record searching, the information you requested was not available.

If you were so certain that there was such deal, it would be helpful if you could provide us the document you've seen so that we can trace the information from that source.

Your truly,