Add a discount to a product with Rules
I finally found a solution:I cloned the rule of the Commerce Sale module:
I created a Component:
Conditions:
-Entity has field:Parameter: Entity: [line-item], Field: commerce_product
-Entity has field:Parameter: Entity: [line-item:commerce-product], Field: field_discount_price
-NOT Data value is empty:line-item:commerce-product:field-discount-price:amount

Actions:Set the unit price to a specific amount 
(Specify the numeric amount to set the price to:Price:Amount
Value:Discount

Then I created a rule:
Events:Calculating the sell price of a product
Action:The previous component
(Data selector:Commerce line items)

Then I used a price formatter and its done https://www.drupal.org/project/commerce_extra_price_formatters
Thanks!