There are several ongoing e-book/e-reader projects in Africa. One of the best-known is Worldreader:
http://www.worldreader.org
There are also several charitable organizations that accept used e-readers as donations, such as E-books For Troops:
http://ebooksfortroops.org/about/
I support both of these organizations with financial contributions and think they're doing great work.
I can't see how the OP's proposal could possibly work, either financially or logistically. There are much better models than an e-reader trade-up program offering a $50 credit. Most people who are interested in supporting this sort of charitable work aren't looking for financial incentives and would probably view this sort of proposal as being rather hair-brained and not likely to succeed.