The solution proposed here will use a fake tax to solve the problem.        

Important facts to keep in mind:        

• The bottle itself does not exist in the stock when filled with beer. We do not want to have two products (bottle and beer 33cl) to sell to the consumers.
• The empty bottle only exists in the stock when the customer brings it back and follow a reversed flow (End consumer > MyCompany > Supplier).
• You cannot use products without price but with a tax only in the POS app (even if it is a fixed tax and not a percentage one). E.g. empty bottle => sales price = 0, tax = 0,50 euros.

Normal flow (Supplier > MyCompany > End customer)

• The bottle and the beer are one single product
• The extra price for the bottle is not an income, but a debt owed to the final customer (and also a debt my supplier owes to my company).

Solution :     

Add a tax to my product 

=> Tax computation = Fixed

=>Set an amount

=> Add the account on which the debt / receivable must be recorded. 

=> Advanced options (on tax form) > Add label on invoice (e.g. "Consigne")

=> If you want your fake tax to be displayed in another section than the main tax ligne (above total): Advanced options (on tax form) > Tax Group, create "Consigne" (e.g. "Consigne")

In this example, I have a receivable account set for any purchase I do so the tax of my bottle is recorded in here. The same principle is applied on my sales tax, with a debt account. 

Reversed flow:

• Empty bottles to refund to the customer
• Stock of empty bottles to hold
• Empty bottles to be sent to the supplier in order to get the refund.

In the POS, when a returnable product come back, I create a negative order line (can be part of a normal order, or only the bottles to refund). 

• My product "empty bottle" is defined as follow :
• Sales price : 0,50    
• Cost : 0,50    
• Tax : 0% (a 0% tax has to be set. If no tax is set, Odoo will use the default tax in invoicing, e.g. 20%.)    
• Accounting, you can use the same income and expense accounts as the ones used for the tax in the normal flow. (to be set on the product or product category if you have a specific one for your returnable products). 

The flow starts when the customer returns products to the point of sale.A negative line is added to the order. The refund can be done in cash or, if your terminal allows it, through a credit operation.

The empty bottles are then in the stock and can be returned to the supplier (credit notes can be useful for the refund from the supplier).