I will let you know when I will be back.

[Rollercoaster1]

I was at home and I texted my friend at his office:
"Can we meet tonight"?

He texted me back:
"I will let you know when I will be back".

I want to ask what would be the difference if he had texted?
"I will let you know when I am back".
Thanks!

 

[emsr2d2]

"I will let you know when I am back" means "I will text you once I arrive [wherever back is]".
"I will let you know when I will be back" means "I will advise you what time I expect to be back".

 

[Rollercoaster1]

"I will let you know when I am back" means "I will text you once I arrive [wherever back is]".
"I will let you know when I will be back" means "I will advise you what time I expect to be back".

Did you mean "inform you" in the second sentence?

 

[emsr2d2]

"Inform" or "tell" work as well as "advise".