Exactly. Morgoth was able to give a good enough appearance of repentance that the Valar gave him freedom. They had offered him the chance after all, and for all that they saw, he was sorry and willing to start anew. But it was not so.
Now if the same happened with Eru, I believe it would play out like this: Eru would offer Melkor the chance to repent. Melkor then has the choice to either truly repent or to fake a repentance(useless) or to deny the offer. Whatever he decides, not only is Eru able to see his real intention, but He has know Melkor's choice before He even offered the forgiveness.