I have no experience with bail, but I assume that most large bail payments are made by check, bank check, or wire transfer.
"Cash" doesn't mean paper money. It means that the defendant needs to actually put up the full $7 million, as opposed to paying a small percentage of it (often 10%) as a non-refundable fee to a bail bondsman for a bail bond.
The government would definitely investigate the origin of the cash, and if Mr. De Guzman couldn't provide an explanation, the government would probably seize it through a civil asset forfeiture
I assume, that the idea on the show is that by the time the government did this, and his bail was revoked, "Mr. De Guzman" would be long gone, and in Mexico, under a different Identity, out of reach of the NM authorities.