This small piece is well written, but I must honestly confess that I didn't enjoy it much. Even willig to see Snape in the bad role here I find that his behaviour seems to be much too petty and spitefull. Even if he might have enjoyed having Lupin leaving thus, I cannot imagine that he would have gloated in this way over it.
As well I find it hard accepting uch a behaviour from Lupin - accepting a bribe at all.