how to apply paypal discount on cart total?

Array(    [return] => http://www.example.com/    [cancel] => http://www.example.com/    [currency] => EUR    [total] => 38.8    [shipping] => 6    [tax] => 2.85    [itemTotal] => 29.95    [items] => stdClass Object        (            [1] => stdClass Object                (                    [quantity] => 1                    [name] => Item one                    [IDFull] => 1                    [priceFull] => 32.8                    [tax] => 2.85                    [price] => 29.95                    [priceWith] => 32.80                    [attributes] => Array                        (                        )                    [ID] => 1                )        ))

This is my sample array for paypal checkout. How to add discount (eg. 20€) on card total price? I have tried many ways, but can't find it out. Thanks!