Does Mr. Greenleaf know about Tom's true personality and why does he pay him the money?

Nearing the end of the movie The Talented Mr. Ripley Tom Ripley is told by MacCarron that he has digged into Tom's past and found out that he never was in Princeton with Dickie thus exposing him as an impostor exploiting Mr. Greenleaf's money. Yet he also tells him that Mr. Greenleaf is taking Dickie's supposed suicide letter seriously and going to pay Tom further money.

I don't completely know what to make out of this conversation, though. Did MacCarron not tell his client Mr. Greenleaf about Tom's lies? If not, then why not? And if he told it, why is Mr. Greenleaf willing to continue Tom's payment and didn't even consider the possibility of Tom being Dickie's murderer, given that Marge is so convinced of it?