Customer : I heard that Option A will be available soon. Could you look into your database and let me know when it will be available ?
Agent : Sure, my assistant here will access the database records. We will let you know when this option will be available.
Assistant (few seconds later) : It will be available starting on Friday.
Customer : I want to buy that Device as soon as Option A becomes available. Here is my contact number. Can you let me know when it becomes available ?
Agent : Sure, my assistant here will take your number. We will let you know when this option becomes available.
Assistant (few months later) : You wanted to know when Option A becomes available. We are happy to inform you that it is available now.
In Scenario 1, the information is already known, and the customer is asking for sharing that information.
In Scenario 2, the information is not yet known, and the customer is asking to be informed as soon as it becomes known.
It is not meaningful to say "We will let you know when this option be available" which mixes up the tenses.