Abdullah ibn Yazid reported that Zayd Abut Ayyash asked Sad about buying wheat against barley. Sa’d asked, ‘Which of these two things is superior?” Zayd said, Wheat.” So, he disallowed that, saying, “I heard someone ask Allah’s Messenger (SAW) about bartering dates for unripe dates. He asked those sitting besides him if the weight of unripe dates reduces when they ripen and it was confirmed to him that they reduce in weight. So, he disallowed that.”
[Abu Dawud 3359, Nisai 4556, Ibn e Majah 2264]
--------------------------------------------------------------------------------