IIRC, she was independently wealthy when she married the President - from her years as an interational model, starting in Milan, then going to Paris. From there she went to New York. I expect she can easily afford it, and would not want any strings attached, anyway.
I dunno. I think they like loan them. The designer will make millions off the fact she “wore them” at a formal event. She doesn’t have some like huge stash of all the gowns she ever wears. It would require a warehouse. For everyday wear she undoubtedly wears her own clothes.