diff --git a/kfet/forms.py b/kfet/forms.py
index 4ba5f7c1..2c01c3c2 100644
--- a/kfet/forms.py
+++ b/kfet/forms.py
@@ -155,7 +155,6 @@ class KPsulOperationForm(forms.ModelForm):
                     "Un achat nécessite un article et une quantité")
             if article_nb < 1:
                 raise ValidationError("Impossible d'acheter moins de 1 article")
-            self.cleaned_data['amount'] = article.price * article_nb
             self.cleaned_data['is_checkout'] = True
         elif type_ope and type_ope in [Operation.DEPOSIT, Operation.WITHDRAW]:
             if not amount or article or article_nb:
@@ -168,5 +167,5 @@ class KPsulOperationForm(forms.ModelForm):
 KPsulOperationFormSet = modelformset_factory(
         Operation,
         form    = KPsulOperationForm,
-        extra   = 0,
+        extra   = 1,
         min_num = 1, validate_min = True)
diff --git a/kfet/migrations/0011_auto_20160807_1720.py b/kfet/migrations/0011_auto_20160807_1720.py
new file mode 100644
index 00000000..97525676
--- /dev/null
+++ b/kfet/migrations/0011_auto_20160807_1720.py
@@ -0,0 +1,19 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import models, migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('kfet', '0010_auto_20160806_2343'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='operation',
+            name='amount',
+            field=models.DecimalField(decimal_places=2, max_digits=6, default=0, blank=True),
+        ),
+    ]
diff --git a/kfet/views.py b/kfet/views.py
index b3eee1f2..7c801b9b 100644
--- a/kfet/views.py
+++ b/kfet/views.py
@@ -402,15 +402,24 @@ def kpsul_perform_operations(request):
     if 'errors' in data:
         return JsonResponse(data, status=400)
 
-    # Pre-saving
+    # Pre-saving (no commit)
     operationgroup = operationgroup_form.save(commit = False)
     operations     = operation_formset.save(commit = False)
 
+    # Calculating amount of each PURCHASE operations
+    # and total amount for operation group
     for operation in operations:
+        if operation.type == Operation.PURCHASE:
+            operation.amount = - operation.article.price * operation.article_nb
         operationgroup.amount += operation.amount
-    operationgroup.save()
 
+    # Filling cof status for statistics
+    operationgroup.is_cof = operationgroup.on_acc.is_cof
+    # Saving operation group
+    operationgroup.save()
     data['operationgroup'] = operationgroup.pk
+
+    # Filling operationgroup id for each operations and saving
     for operation in operations:
         operation.group = operationgroup
         operation.save()