Is exchanging a luxurious palm tree for two non-luxurious palm trees considered usury (riba), by analogy with the Prophet's (peace be upon him) prohibition of exchanging one saa' of good dates for two saa's of bad dates?

There is no harm in selling one palm tree for two palm trees due to the absence of the 'riba al-fadl' (usury of surplus) علت (reason/basis) between them. Palm trees are not measured by volume (makīl), nor are they eaten (maṭʿūm), nor are they staple foods that can be stored (qūt mudkhār). These are the علل (reasons/bases) for riba in the six categories mentioned in the prophetic hadith: "Gold for gold, silver for silver, wheat for wheat, barley for barley, dates for dates, and salt for salt, like for like, equal for equal, hand to hand." As for selling palm trees and whether the land is included with them or not, the default is that the land does not follow unless explicitly stated in the contract or if custom dictates it.