What would prevent a sponsor paying more to help secure certain players ?
There wont be a cap on how much a sponsor could "invest" ?

There is if it goes in the players pocket. That's the kind of deal that gets you points docked.

To get away with that you'd have to use someone not associated with the club......and I'm sure they'd still investigate it.

Would it matter who paid as long as you stayed under the cap?

No it wouldn't.....and I think we have loads of cap room.
But that's not what Wrencat said. I was answering the bit about not affecting the cap.

Payments made direct/indirectly to a player are still subject to Tax & NI and therefore count on the salary cap, if not then there is also an element of tax evasion which would land all parties in the do do if found out

Sponsorship is just another income stream and will be included against the cap.