Moderator should be allowed to charge a fee that is lower than what was agreed upon

[lunokhod]

In moderating my first dispute with a vendor, I wanted to do this for zero fee. When clicking on the button to close the dispute, however, I was only given the option to split the funds between the vendor and buyer: the moderator fee was already removed. In my opinion, the moderator should be able to charge a fee that is lower than what was agreed upon (but not higher).

Client: OSX commit 030304053bf660723be6d54798294f426d24f11d Server: OSX commit 96cbc5595affcb437ecadbdab0800e4c40ce12a6