Personally, i would get them to send both. especially if the ballscrew and nut are Chinese, Replacing a nut was always an issue, because the Chinese buy the reject screws from the big factories like TBI in Taiwan and then make their own nuts to suit the screw. But they get rejected for a number of issues like the root diameter is not within specification.
So you end up getting a replacement nut and it does not fit correctly on the screw and is either too loose or too tight.
Get them to match the nut to the screw and test its straightness prior to sending.