[Django]-Djano annotate with subtract operation returns None when the subtrahend is None

I haven’t tested this, but Coalesce should do the job when the sum aggregation is None.

from django.db.models import Sum, Value

qs = Billing.objects.filter(bill_batch=prev_batch, balance__gt=0) \
    .annotate(
        payment=Coalesce(
            Sum('paidbills__amount_paid', filter=Q(paidbills__pay_period=batch.period)),
            Value(0)
        )
    ) \
    .order_by('reservation__tenant__name', 'reservation__apt_unit') \
    .only('id', 'bill_batch', 'reservation')