[FIX] purchase: Purchase report wrong qty_to_be_billed

Steps to reproduce the bug:
- Let's consider product P with purchase_method = 'purchase'
- Create a purchase order PO and puchase 20 units of P to a vendor
- Confirm PO and create a vendor bill VB the 20 units of P
- Validate VB
- The invoiced quantity on PO is 20
- Go to the Purchase report and analyse the qty_to_be_billed

Bug:

The qty_to_be_billed was still equal to 20 instead of 0

opw:2091264

closes odoo/odoo#39493

X-original-commit: b00febedef7826f593bb07c7896595ecce8cae32
Signed-off-by: Simon Goffin (sig) <sig@openerp.com>

addons/purchase/report/purchase_report.py

@@ -92,7 +92,10 @@ class PurchaseReport(models.Model):
                     sum(l.product_qty / line_uom.factor * product_uom.factor) as qty_ordered,
                     sum(l.qty_received / line_uom.factor * product_uom.factor) as qty_received,
                     sum(l.qty_invoiced / line_uom.factor * product_uom.factor) as qty_billed,
-                    sum(l.product_qty / line_uom.factor * product_uom.factor) - sum(l.qty_received / line_uom.factor * product_uom.factor) as qty_to_be_billed
+                    case when t.purchase_method = 'purchase' 
+                         then sum(l.product_qty / line_uom.factor * product_uom.factor) - sum(l.qty_invoiced / line_uom.factor * product_uom.factor)
+                         else sum(l.qty_received / line_uom.factor * product_uom.factor) - sum(l.qty_invoiced / line_uom.factor * product_uom.factor)
+                    end as qty_to_be_billed
         """ % self.env['res.currency']._select_companies_rates()
         return select_str
 
@@ -135,6 +138,7 @@ class PurchaseReport(models.Model):
                 line_uom.uom_type,
                 line_uom.category_id,
                 t.uom_id,
+                t.purchase_method,
                 line_uom.id,
                 product_uom.factor,
                 partner.country_id,