Hi bro, 

You can solve this problem by following these steps:

1. Create a new class that inherits from purchase.order.line. 

2. Copy and paste the code down below inside the inherited class. 

3. Vote positif :). 

The code:

@api.onchange('product_qty', 'product_uom') 
def _onchange_quantity(self):
    price = self.unit_price
    sup = super(purchaseOrderLine,self)._onchange_quantity()
    self.unit_price=price
    return sup