earnest money, silk label on boo
贉 centers on a financial deposit or label: earnest money paid in advance, or a silk label attached to goods. The meanings are loosely connected through the idea of a marker of value or ownership.
贉 combines 貝 (shell, money) with 覃, which likely contributes the sound. The exact historical development is uncertain.
The left side 貝 represents money or valuables, and the right side 覃 looks like a container with a lid. Imagine placing earnest money inside a labeled container: 贉 is a deposit or label.
For タン, picture a tan-colored silk label tied to a money pouch: tan -> タン, and the label marks the earnest money.