I have spoken with my manager about the possibility of updating the plan, and/but we are unable to do this [there is not this capability*] at the moment. Any changes should be submitted by means of an official notice by/from your Management, to **allow for/[share] the possibility that agreement on the target are is not reached with our Management. 

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*not wrong, but rather formal

**try to reach agreement with our Management on the proposed target(s)

I spoke with my manager about the possibility of updating the plan but this is not possible at the moment. Any changes should be submitted in an official notice from your Management in order to let us know if the target may not been reached by our Management. 

<em>I don't really know enough context to have a good feeling for the meaning of your note.  I cannot guarantee that my correction means exactly what you intended.</em>

<em>The first sentence has a standard formal style, but the second is very formal and impersonal.  </em>

<em>The usage of "Management" is unusual.  I am not sure which people you mean.</em>

<em>Italians use the word "possibility" a lot but this word is quite formal, and not so often used in English. We normally use "can", "could", "may" instead, depending on the shade of meaning.</em>

<em>Kind regards, Michael </em>