Dear Hiring Manager,
Thank you for your letter of March 10. After careful consideration, I regret to inform you that I am unable to accept your refusal to offer me a position in your department.
This year I have been particularly fortunate in receiving an unusually large number of rejection letters. With such a varied and promising field of candidates, it is impossible for me to accept all refusals.
Despite your companies outstanding qualifications and previous experience in rejecting applicants, I find that your rejection does not meet my needs at this time. Therefore, I will assume the position in your department this August.
I look forward to seeing you then.
Best of luck in rejecting future applicants.