As Chas has pointed out, even if the Barony associated with Craighall-Rattray transferred with the sale of the land (which it probably did) Lachlan Rattray would still be styled as:

Rattray of Craighall Rattray as that is the designation of the chief of the Rattrays.

The only exception to this would be if the new owner of Craighall Rattray also bore the surname Rattray. In that instance some accommodation would have to be reached to avoid confusion-- it would be most likely that the chief would assume the style of either Rattray of Rattray, or Rattray of that Ilk, or perhaps Rattray of Rattray and Craighall. He could as easily stand on his dignity causing the new owner to modify his territorial designation-- something that the Lord Lyon would carefully consider before making any decision regarding the name and arms of the new owner.

In any event, merely divesting himself of the property does not cause Rattray to loose his chiefly titles, or his position as Chief of the Clan Rattray.