Some issue with Rules & PayPal

Hello,

I created a rule for these summer sales, called simply 'sales', that gives an -25% discount on the order.
This rule is apply on the cart (I bypassed the following problem by applying this rule on the products instead).

The problem is: the discount is a negative amount, and PayPal don't accept it returning the following error : incorrectly formatted item amount.

The sandard product amount format is x.x and -x.x for the rule. Why does the rule triggered an error in PayPal ?

Have you got any idea concerning this issue ?

Sincerly.