Good idea! It would certainly add some fun & spirit of rivalry to multi-player games.
I don't think any limitation per day would be needed. You can only win so many multi-player games per day (they tend to move pretty slowly, especially foursomes, although these usually disintegrate pretty soon due to quitters) . Let's say someone wins 20 multiplayer games in a day, which is a huge stretch. So what? Such a player would only win 5 credits for that day-long effort, and you can buy pretty much nothing at all for 5 credits ($0.05) in the ProShop.
I'd maybe go further & say give half a credit to the winner of each multi-player game; a quarter-credit to the second-place finisher; and a tenth of a credit to the 3rd place player. (Assuming 4 players started the round; the last-placed player would never be rewarded with anything.) This, too, might help reduce the amount of quitters in multi-player. A tenth of a credit isn't much of a reward, but it might still sway some folks & persuade them not to hit that Exit button just because of a string of bogeys. Even symbolic wins of tenths of a credit can be motivating.
Also, since the winner of a foursome would get half a credit, and the winner of a multi-player with 2 players would only get a tenth of a credit, this would motivate game hosts to launch foursomes rather than threesomes, etc. That would definitely be a good thing 
Plus, lower-tier players wouldn't perhaps be booted from multi-players so frequently; one might want to "keep them" as "easy prey" for the presumed multi-player game winner. 