simple sample here:
Dear whowhowho,
I write to dispute your charge listed in your letter dated date1..... in its entirety.
On or about date2, a leak was found in unit xxx on the 11th floor with an unknown source of the leak. The 管理处 hired your company (ABC) to troubleshoot and discovered the leak was caused by unit yyy on the 13th floor, which indirectly caused the stain in my unit zzz on the 12th floor as well. Mr whowhowho of 管理处 witnessed and was fully aware of the incident then.
Please cancel your claim to me immediately. I wish you find the right party to invoice for your service.
whowhowho