For me it just seems as a viable excuse a club can make to validate their claims they don't "pay" players. I'd like to know if these potential sponsors are actually aware that the money they are "sponsoring" is in fact going straight into player A's pocket, I'd find it hard to believe that they actually do!
The club isn't paying players whether you or anyone else thinks it's a viable excuse or not.
Now wether third party funded win payments are kosher is a good question for any amateur competition. I'm sceptical that no matter how convuluted the route is to the player, $ incentives in a nominally amateur league is 'fair play'.