Thanks Ermin Trevisan for your help, but finally I discovered that when installing "Sale Discount for Total Amount" module, the total amount of my sales order will be rounded. When I uninstall it, the total amount is right. I would have a dsicount in the total amount of my sales order, that's why I insttalled this module. Is there any solution to get a total discount without adding "Sale Discount for Total Amount" module ? 

Thanks in advance.

Define the decimal accuracy and rounding in the settings of your currency.