Hello Cybrosys, thank you for your answer
I've been poking around for an answer then I found this thread https://www.odoo.com/forum/help-1/purchase-returns-in-v14-186762
and I think this is the "official" answer from Odoo for now
I guess we'll have to wait or do some customization here haha
But anyway, thanks for checking out :D
In FIFO, when a product leaves a stock the 'First In First Out' rule will be applies. The first value “in” is the first value “out”, no matter the storage location, warehouse or serial number.
Whenever the products leaves stock, this rule will be applied.
In your case, you may be created a return transfer and based on this rule first in will be the first out.