They could happily provide a laundering service of sorts. If the goblins get suspicious of where Harry got the muggle money (which he will supposedly get via one of many ways outlined on this thread), he could just say that the Grangers gave it to him to rescue his daughter-- He earns money through them (stocks in their name, or whatever), and they turn around and give it back to him to trade into galleons for the Hermione-Rescue Debt.

I doubt the goblins would look into how MUGGLES managed to earn so much money. Especially if they already seemed to be relatively well-off.