I regret to inform you that we will decline your proposal.

아쉽게도 당신의 제안을 거절해야 할 것 같군요.

• I am terribly sorry to say that we cannot accept your proposal.
• I'm afraid that we cannot accept your request to extend the due date this time.
• Unfortunately, we cannot shorten the delivery time.
• Regrettably, I must decline your request at this time.
• I hate to say this, but I am unable to join the conference.