I'd be very surprised if the budget was used up "for the year". Wal-Mart stores are each given a set amount for each month. Often they have foolishly spent it all in the first week or two instead of stretching it out over the month like they are supposed to. But each store, from my understanding is given an amount each month . . . not a lump sum for the whole year.
It is more likely that each Wal-Mart store is under orders to limit donations to their immediate retail area, to maximize community good will, and to ensure positive press in each store's immediate area newspaper and other media. That "the budget is used up for the year" sounds like an excuse to not donate to a distant third world country.
Try this link to contact the corporate office directly http://www.walmartfoundation.org/