Very risky trade on both sides. But I think more on your side because if Curtis Martin stays healthy he will put up simliar numbers as Alexander but you will lose out because you traded Manning.
I would try a different approach. Why don't you try to package Tiki. Testeverde and Ismail to get a top ten RB? I think your team would be solid at that point because you are just a little thin in the RB position. This would also allow you to keep Manning.
My vote is to hold off on that trade and try a different approach. Who knows maybe that guy will come down in demand alittle bit while you look around.