Subpoenas to a Non-Target

I am on a number of discussion lists as a lawyer.  On one of them, the issue of responding to subpoenas by a non-target was discussed.  Essentially, the issue posed was this:  Non-target receives a subpoena for documents and it is (as most are) extremely broad.  So you call the AUSA who issued it and the USA tells you over the telephone what the government is looking for and verbally agrees to accept a limited response (but reserves the government