change_credit should add positive credit, not substract it

cash/models.py

@@ -28,7 +28,7 @@ class Account(models.Model):
 				instance.account.save()
 
 	def change_credit(self, amount, subject, desc):
-		self.credit -= amount
+		self.credit += amount
 		self.save()
 
 		transaction = Transaction(account=self, subject=subject,
@@ -57,7 +57,7 @@ class Account(models.Model):
 
 				desc += '%dx %s\n' % (amount, product.name)
 
-				self.change_credit(total_value, BUY_SUBJECT, desc)
+				self.change_credit(-total_value, BUY_SUBJECT, desc)
 			return True
 		else:
 			return False