Selling by weight in variations - how to set up?

Selling by weight in variations - how to set up?

I am trying to figure out on how to set it up so that it will work in Books and Inventory
We buy tea in 1000g bags. So the units could be either g or kg

In my wooocommerce shop I have the product with 4 different variations, eg., 50g, 100g, 250g and 500 g
Each variation has its own SKU and EAN (which I need for Google Merchant and Ebay)

It looks like this:

Tea Main product - SKU 1000
50g - SKU 1000-50
100 g - SKU 1000-100
250 g - SKU 1000-250
500g - SKU 1000-500

So far, so good.
In our brick and mortar shop we usually have the same measurements. From time to time, we are selling in different quantities, eg. 80 g. This can happen when a customer wants a caddy filled that can only take 80g of a particular tea.

I have no idea how to set it up.
In Inventory as well as books I tried with the variations, but that did not work.
Whenever I add a variation to an invoice, it counts as 1 unit, no matter if g or kilo, it is always 1. and then the price of the variation.

Which means, that the weight would not be reduced properly
The main stock would be 1000g and if someone buys 3x 100g, then 300g would need to be reduced from the stock and not 3.

Can someone please explain in simple terms on how to set it up?

Thank you in advance