Informing or informed us?

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I am a filipina and is always being corrected by a singaporean supervisor pertaining to my letter-correspondence. I am working in singapore and is in charge of of admin matters in our office. Sometimes i felt that i am the one in the right side but i ca't do anothing but to follow as I am only a subordinate.
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Can you tell me if this sentence is wrong: " We refer to your letter dated 12 Sept. 2007 informing us of applicable penalty interest for late submission of CPF contributions. "
My supervisor said "informing us " is not the right phrase to be used as the letter was received in the past. He wants to use "'informed us", which as per my grammar knowledge is wrong. Please enlighten me on this.