From the sales app, If you select Discounts you will be able to add a discount as a percentage on a sales order line. 

If you need to add a number-amount as a discount

Go to sales-configuration-settings- select discounts, loyalty & gift card.

Create a product: 

Create a new quotation: 

Add your products and the discount (negative number) in separate order lines. 

If you have Mexican localization and you need to create an invoice:

1. Add UNSPSC Category

1. Create a new parameter

Create a new parameter 

1. Key: l10n_mx_edi.manage_invoice_negative_lines

2. Value: True

3. Create the invoice of the order sale and the XML will be shown as this.

This is to say, you won't have any problem with the SAT.