Received $600 for a $1200 consulting gig: need advice

Perhaps you can help me with a situation.  I had an agreement with an
advertising agency for 15 hours a week.  Because he was agreeing to
buy 15 hours per week until Jan 1, I gave him a discount: $400 for 15
hours a week instead of $600, paid in advance, like everyone else.  I
get there yesterday, he signs the agreement, and instead of $1200 (3
weeks for November = $1200) gives me a check in front of his secretary
for $600 and says "I'll give you the other $600 in 2 weeks".  That
essentially means that instead of discussing this with me, he made the
unilateral decision to just pay 1.5 weeks and in 2 weeks will pay the
remainder.
 
He therefore has paid for this week and half of next.  In 2 weeks he
will be behind.  I don't work behind. I was miffed but didn't want to
make a scene in front of his secretary. On the way home I became
angrier, thinking, hey he didn't even discuss this with me, he treated
me like an employee instead of a consultant, I felt disrespected for
being treated that way and angry at myself for not figuring out a way
to say something, and in 2 weeks (11/26) he will be 1.5 weeks behind
AND December for 5 weeks ($2000) will be due one week after that
check.
 
Got home and there was a one line email from him sent at 1 PM after
I'd left my house, saying this:
               I am paying half today so invoice me for that. 
 
Not sure what I should do.  On the one hand, perhaps he's worried that
I won't work out and he'd be out the $1200 for nothing.  ON the other
hand, perhaps they have money troubles and I don't want to get into
the middle of having people owe me (one client owed me $25000 and I
only collected $2000 of that before they went belly up which is why I
started charging up front to begin with!).  SO, I have the following
options and perhaps you can think of others:

1.  Say nothing for now, figuring he's checking my work out. WOrk my
15 hours this week and 7.5 hours next week and submit an invoice next
Tuesday and not work until that is paid.
2.  Send an invoice for $1200, mark off $600 as paid and the other
$600 due next Wed.
3.  Send an email asking why he did this and explaining the policy.
4.  Send an email explaining the policy and not ask.
5.  Other ??
 
I'd appreciate your input.  I don't want to alienate him, as I want to
work this client.  However, I don't want to set up a situation in
which he routinely makes his own decisions about paying me, a
consultant, and treating me disrespectfully in that way.  ON the one
hand, perhaps he's worried I won't perform.  On the other hand, at our
set up meeting 2 weeks ago I made it clear that I worked on advance,
didn't work on arrears, he said I'd never Jew you down (terrible
verbage on his part even if we are both Jews) and then he asked for
the discount and THEN did this, showing me he does intend to "jew me
down".  SO, I'm not sure how to respond.  I need to respond in a
respectful way that doesn't allow him to be disrespectful of the
agreement and of me, yet not to appear greedy or inappropriate.
 
What do you think I should do or say? 
Really appreciate any input.
 
Anne