fief
埰 centers on a fief: a piece of land granted by a lord to a vassal, often marked by an earthen mound or boundary.
埰 combines 土 (earth) with 采, which likely contributes the sound and may relate to marking or dividing land. The exact historical development is uncertain.
The left side is earth 土, and the right side 采 looks like a hand picking something from a tree. Imagine a lord picking a piece of land from the earth to grant as a fief.
For サイ, picture a sigh of relief as a vassal receives a fief: sigh -> サイ, and the land is marked with earth.