So, at any time, the server has

- fully signed multi-sig transaction (value of A + B)
- refund transaction that pays A to the client and B to the server

The client has

- fully signed multi-sig transaction
- refund transaction that pays the entire multi-sig back to the client (time locked though)

As long as the server broadcasts its 2 transactions with sufficient time before the locktime, it is guaranteed to get B coins.

The client can pay more by sending a new refund transactions with lower A and higher B.

What is the default safety margin?  With a 24 hour channel, is it 12 hours "active" and then 24 hours of lock time?

Anyway, sounds cool.  What exact use case are you thinking of?   Something like video streaming where you pay a small amount every 20-30 seconds?

Micropayments would be cool for "like" type situations, where you can setup your browser to pay to a link.  However, that wouldn't be supported by this, unless you do a two level system.  Your brower connects to your micropayment service and it handles the actual transfer to the web site.