how to override django save method to update some field dynamically?

Solution One:

I think instead of changing in Ledger model, you should change in Expense model, like this:

class Expense(models.Model):
    ...
    def save(self, *args, **kwargs):
       self.payment_option.amount_to_pay = self.payment_option.amount_to_pay + self.amount_to_pay
       self.payment_option.save()
       super(Expense, self).save(*args, **kwargs)

Solution Two:

But to be honest, Solution One does not seem good to me. Reason is that you are saving same data in 2 places(in both expense and ledger). Instead, it should be once, then the amount_to_pay value in Ledger should be calculated dynamically. Like this:

from django.db.models import Sum

class Ledger(...):

     @property
     def amount_to_pay(self):
         # I am using a property method to show the amount_to_pay value.
         # FYI: in this way, you need to remove amount_to_pay field from Ledger model
         return self.opening_balance - self.expense_set.all().aggregate(a_sum=Sum('amount_to_pay')).get('a_sum', 0)

In that way, with each ledger, the value amount_to_pay will be dynamically calculated at runtime. For example:

 for l in Ledger.objects.all():
     l.amount_to_pay

Solution Three:

If you are wary of making DB hits with each l.amount_to_pay(as it calculates amount_to_pay from DB dynamically) from previous solution, then you can always annotate the value. Like this:

For this solution, you need to change your Expense model and add a related_name:

class Expense(models.Model):
    pay_from = models.CharField(max_length=200)
    payment_option = models.ForeignKey('Ledger', on_delete=models.CASCADE, related_name='expenses')

Then use that related_name in query like this(FYI: You can't keep def amount_to_pay(...) method in Ledger model for the following usage example):

from django.db.models import Sum, F, ExpressionWrapper, IntegerField

ledgers = Ledger.objects.all().annotate(expense_sum=Sum('expenses__amount_to_pay')).annotate(amount_to_pay=ExpressionWrapper(F('opening_balance') - F('expense_sum'), output_field=IntegerField()))

# usage one
for l in ledgers:
   l.amount_to_pay

# usage two
ledgers.values('amount_to_pay')