This is not finders keepers. Elgin didn't unearth these treasures from some hidden tomb long buried, long lost. The Greeks kind of knew where the frieze was when they told him he could have a look at it. And it doesn't matter if the theft may have saved them from some degree of damage. Although it does seem likely that the sculptures would've suffered some further incremental defacement between the theft and Greece's independence, it's a fact that Elgin's salvage operation - in the course of hacking the figures free from their housings - smashed and destroyed much magnificent masonry and cornice work, along with much of what was left of the building. Whether the damage that we know was done offsets the damage that we think might have been done is an unprovable point, and moot. Either way, it doesn't justify keeping them now.
Hey, I gave it to him already, and I give it to him again! It was one hell of a stunt, and a fine prank besides. But a joke's a joke, guys. It's time to make good on this deal. Execute the original permissions. Take some casts. Make some drawings. And then give the damn rocks back.
"Cold is the heart, fair Greece! that looks on thee,
Nor feels as lovers o'er the dust they lov'd;
Dull is the eye that will not weep to see
Thy walls defac'd, thy mouldering shrines remov'd
By British hands, which it had best behov'd
To guard those relics ne'er to be restor'd.
Curst be the hour when from their isle they rov'd,
And once again thy hopeless bosom gor'd,
And snatch'd thy shrinking Gods to northern climes abhorr'd."
That's Lord Byron. I don't go around quoting Lord Byron often or without strong cause, you may be assured of that.