You just want to make a receipt of the empty bottle (component product and not the product being sold) .It may appear simple but there can be more use cases for example like deliveries can be partial, so can be the returns as well.You would require to read the code for handling of deliveries from a sales order and try to amend it for your requirements related to associated returns.

Thanks

Vishal

Hi Vishal, thanks for the reply.

Yes, that's what I'm currently doing as a workaround: creating a receipt manually for the empty bottle coming in. It does what I want in terms of tracking what's coming in from the customers but it does not associate the IN transfer to the Sales Order, even if setting the SO number to the Source Document of transfer. It does not create a link between SO and Transfer. 

Ultimately, there are two things I want to learn to do:

1. How to modify the SO's save so it creates two transfers(OUT & IN)

2. The Delivery button should display count "2" instead of "1", just like having the 2 Steps(Pick + Ship) enabled. 

One thing to add, as I did not mention in my question, is that I'm using Odoo 11 CE.