I just payed for a months subscription in order to mod the credits in the old Forza 3. Long story short it doesn’t work. I import the save, use the mod tool then inject it back onto the xbox hdd and try to play it, the xbox simply creates a brand new save secondary to the one I modded. 
Why?
Note: I modded Forza 4 years ago so I know it’s supposed to work.
Does the one you modded still show up or has it been corrupted?
It shows up in on the xbox and in the explorer. I tried it a bunch of times, I even loaded my original save back on the xbox hard drive and it does the same thing.
It did the same thing when I tried to mod it with xbox 360 tools the other day, which is why I payed for this program again.
I would check to make sure the save has the correct profile ID if it doing it even with the original save. You may also want to just try leaving it on a flash drive instead of moving it back to the xbox HDD.
I tried leaving it on the flash drive as well, same thing.
Save you played the game any or are you starting fresh then trying to mod it? It has been a long time since I played Forza 3 and I unfortunately don’t have it anymore so I’m just trying to think of things that could make a difference.
I played it for less than an hour and had 14 cars, 33**XP and was on level 3, I then tried to mod that save. Nothings updated on the game/console to prevent it from working anymore has it?
I doubt it seeing as they haven’t released any DLC for it in a while. You can try clearing title updates from the game to see if that makes a difference.
I can’t see title updates to delete anyway. I just started fresh again and it still didn’t work, New Save > Played to level 3 > moved save to USB > Plugged into PC > edited with Horizon no problem (200,000,000 credits, lvl 47) > plug USB back into 360 > Move save to HDD > Start game > creates a new save, doesn’t register the modded save.