You should write as under;
Mr. Name
Company
Address
Subject: Donation
Dear Sir/Madam,
Thank you for writing me a letter. Your organization is doing a noble cause to bring happiness in the lives of (you can mention here the function of the company, means for what they are asking for donation). I am sorry to inform you that my company is already donating to other organizations. I will try my best to send you some donation in the near future.
Thanks & Regards,
Your Name,
Address.
Mr. Name
Company
Address
Subject: Donation
Dear Sir/Madam,
Thank you for writing me a letter. Your organization is doing a noble cause to bring happiness in the lives of (you can mention here the function of the company, means for what they are asking for donation). I am sorry to inform you that my company is already donating to other organizations. I will try my best to send you some donation in the near future.
Thanks & Regards,
Your Name,
Address.