Error 'unsupported operand type(s) for +: 'UserBalanceChange' and 'int''

I want to make multithreaded transactions in django, when the payment of one user is divided between the second and third user in different proportions. I wrote the code. It works for two users, and when a third user appears, an error appears, although the code is exactly the same as for two users.
my views.py:

    @transaction.atomic()
def transaction_file(request):
    if request.method == "POST":
        try:
            buyer = request.POST.get('buyer')
            seller = request.POST.get('seller')
            admin = request.POST.get('admin')
            amount = request.POST.get('amount')

            buyer_obj = UserBalanceChange.objects.get(user = buyer)
            seller_obj = UserBalanceChange.objects.get(user=seller)
            admin_obj = UserBalanceChange.objects.get(user=admin)

            with transaction.atomic():
                buyer_obj.amount -= int(0.8*int(amount))
                buyer_obj.save()

                seller_obj.amount += int(0.8*int(amount))
                seller_obj.save()

                with transaction.atomic():
                    buyer_obj.amount -= int(0.2 * int(amount))
                    buyer_obj.save()

                    admin_obj = admin_obj + int(0.2 * int(amount))
                    admin_obj.save()
                # messages.success(request, 'Транзакция успешно проведена')
                return HttpResponse('Транзакция успешно проведена')

        except Exception as e:
            print(e)
            # messages.success(request,'Что-то пошло не так!(')
            return HttpResponse('Что-то пошло не так!(')

    return render(request,'chat/files/chattransaction.html')

my models.py

class UserBalanceChange(models.Model):
user = models.ForeignKey(User,on_delete = models.CASCADE, related_name='balance_changes')
amount = models.DecimalField(verbose_name='Amount', default=0, max_digits=18, decimal_places=2)
datetime = models.DateTimeField(verbose_name='date', default=timezone.now)

def __str__(self):
    return f'{self.user.username}'

my html:

<form method="POST" style="width: 50%;">
                      {% csrf_token %}
    <form method="post">
      <div class="form-group">
        <label>Покупатель</label>
        <input class="form-control" name="buyer" type="text" placeholder="Покупатель">
      </div>
        <div class="form-group">
        <label>Продавец</label>
        <input class="form-control" name="seller" type="text" placeholder="Продавец">
      </div>
      <div class="form-group">
        <label>Админ</label>
        <input class="form-control" name="admin" type="text" placeholder="Админ">
      </div>
        <div class="form-group">
        <label>Цена</label>
        <input class="form-control" name="amount" type="text" placeholder="Цена">
      </div>
      <button type="submit" class="btn btn-primary">Купить</button>
    </form>