Gentlemen:

Our contract entered into on (date) specifies that
we will deliver (specify goods) .

We regret to inform you that the goods herein referred to
are unavailable because (set forth reason) .

We are hereby requesting your authorization to substitute
the following product in lieu of the one we cannot obtain:
. If you are agreeable to this, would you
please acknowledge your acceptance by letter.

We will look forward to your response.