Hey Paul,
One solution I have used for this in the past doing a full Shopify setup for both the POS and the Online store is to create a customer for Mr.Bar and then sell bottles at a %100 discount to that customer to keep track of bottles moving from customer inventory to the bar inventory. The reason I like this is then you can account for the dollar value in inventory by looking up the Mr. Bar customer. You can also gate the discounting feature to managers on the POS Pro so you can sleep a little easier knowing this operation is only being completed by staff of a certain level.
Where I have found the POS to be lacking for a service business is Shopify doesn’t like an open table type of invoice its much more set up to take money and close an order rather than keep an order open and continue adding to it. I do kind of like this middle solution of using the Shopify POS to take inventory from Shopify/ bottle sales for customers and then use a POS system more suited to your industry for the actual table/ bar service side of things.
Let me know if you have any questions as you explore the options here!
-iain