Leviathan could make her go and they both knew it. Rather, he could drop Mammon - or anyone else, for that matter - down the long, dark, seemingly bottomless rabbit hole until she found herself back in pleasant, sunny, perpetual summery Hell. It wouldn't be a light, comfortable landing, either.
He took a step towards her, his stance passive, his orders absolute. He had a few hours left and if she wanted to exchange spiteful words or bitter blows he was fine with that too. They had until the End of Days to settle their scores, anyway.
"Take what you need and leave, or be sent back. Either way you cannot stay here." Not a threat, no malice or anger - an ultimatum of sorts that, in the end, was in accordance with Lucifer's wishes.