As i understand from you this case happened because the client sent email for requested the time changing from you and you tried to interact with the person concerning but could not get them that why you have answered like that.
As i told you in the telephone talked, next time don't commitment with the client without consulting the authorized person.
Dear Khun Varghese,
If this is a case i will take responsibilities on my fall.
Gap,Thank you for you try to get back to client quickly but next time don't hurry-up.