This could be a central part of how this case plays out, but it's far from certain that Trump didn't turn that offer down. He has been extremely protective of "his boxes" so there is a good possibility he just wanted them under his control, in his house.
“...it’s far from certain that Trump didn’t turn that offer down.”
I’ve heard Trump say several times that the National Archives didn’t show any interest in what he was taking. He said the boxes were stacked up in the driveway for hours for anyone to see, and they never came over to look into the situation. Given the fact that the NA has been politicized to the left, I still believe it was a setup.