If I understand correctly, the problem we are trying to solve is how to implement Buy X get Y free.

I am assuming this means that if a customer buys more than X, they still Y free. 

Make sure you have pricelists enabled:

- Go to Sales > Configuration > Settings

- In the 'Pricing Section' make sure 'Pricelists' is checked and check the 'Advanced Pricelists' option.

Make changes to the Pricelists:

- Go to Sales > Products > Pricelists

- Add rules for each product/product category

For your situation (buy 5, get two free). You can consider it a quantity discount.

For example, Item A normally sells for $100. If customers buy 5 (or more) then they get 2 free.

If there are no matching rules in the pricelist, then Odoo will set the unit price to the 'Sales Price' field on the product template. Therefore, we only need to set one rule to apply the quantity discount:

- The line for the product on the pricelist would be:

    Computation: Formula

    Based on: Sales Price

    Extra Fee: $-200

    Apply on: Product (select the product here)

    minimum qty=5 

This means that when >5 of the product are ordered, a discount is applied that subtracts the price to two items from the total.

Your actual use case may require something slightly different from above, but I hope I have given you a good starting point.