Checking a chest for a specific item

I’ve been watching video tutorials for using command blocks and browsed a bit on here before forming this question.

I’ve been trying to make a shop where a player puts a few emeralds, let’s say 8, in a chest and they receive something, let’s say an obsidian block. After a good amount of experimenting, the command that has come close to my solution is:

execute if block ~ ~ ~ chest{Items:[{id:"minecraft:emerald", count:8b}]}

I also used the variation: execute unless block...

I have a button to press it but it only registers the chest; regardless of whatever is inside of it (even executing if the chest is empty). I’m on 1.16, so I don’t think the "/testforblock" command (as seen in other questions) works. Is there something I’m missing?