This is just nonsensical.
Those 3 Dummy Halves might be well sought after now, but that really wasn't the case to the same extent when they signed their current contracts, so their value on the open market today is irrelevant in Melbourne's current salary cap because they are not on the open market, they're still locked up under existing contracts at Melbourne.
Payne Haas is also getting paid a shitload less than he's really worth on the open market but luckily for us he signed a 6 year deal before he'd really cracked first grade, so his current hypothetical value is irrelevant because he gets paid what he agreed to at the time of signing.
Harry Grant and Brandon Smith are no different to Haas in this example. If Melbourne choose to release them it will be because of player welfare and contentment, and nothing to do with not being able to fit their current contracts under the salary cap.