#238 Fix

* adds casting from Decimal() to primitive float for proper calculation

compensation/models/eco_account.py

@@ -118,8 +118,15 @@ class EcoAccount(AbstractCompensation, ShareableObjectMixin, RecordableObjectMix
             intervention__deleted=None,
         )
         deductions_surfaces = deductions.aggregate(Sum("surface"))["surface__sum"] or 0
-        available_surfaces = self.deductable_surface or deductions_surfaces ## no division by zero
-        ret_val = available_surfaces - deductions_surfaces
+
+        available_surface = self.deductable_surface
+        if available_surface is None:
+            # Fallback!
+            available_surface = deductions_surfaces
+        else:
+            available_surface = float(available_surface)
+
+        ret_val = available_surface - deductions_surfaces
 
         return ret_val