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 subtitute
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.