Maybe you should instead donate any leftover money to CFAR, so as to avoid the donor illusion.
He could start a giving war by setting up a fund for AMF and a fund for CFAR. At the end of the year, he could donate a total of 30% of his income to both charities, splitting the fund in proportion to how much each charity raised in the giving war.
To get a real war going you want some advantage to escalation. Simply spitting it in proportion to the total giving diminishes the value of a marginal dollar. Maybe an all-pay auction (like a dollar auction) would be better: whichever of AMF and CFAR got more donations would get the full match from me. Among rational mixed-strategy-utilizing donors this should result in $X moved by an $X pool on average, but people being imperfect I would expect to see more than $X moved.
Giving money to effective charities is one of the ways we can have the biggest positive impact in the world. If you've been thinking about giving to GiveWell's top-rated charity, the Against Malaria Foundation, but haven't gotten around to it, perhaps this will help: make a donation of up to $2K and I will match it 100%. I'll match up to $20K this way.
(Any money left over at the end of the year will also go to the AMF. That makes this a donor illusion, in that you're not actually affecting the amount I give them this year.)
I made the same offer on my blog