This is a very naive approach to the issue and it's a race condition away from being a day+ debugging nightmare for you.

def increase_stock_quantity(self, amount):
    self.quantity_in_stock += amount
    self.save()

def decrease_stock_quantity(self, amount):
    if amount > self.quantity_in_stock:
        raise ValueError('The Amount is Larger Than the Quantity in Stock')
    self.quantity_in_stock -= amount
    self.save()

1. use locks (.select_for_update())
2. (or) use query set self.objects.filter(id=self.id).update(quantity_in_stock=F('quantity_in_stock') + 1)

Also make sure you wrap this inside a transaction.

def save(self, *args, **kwargs):
    if self.issued_quantity > self.item.quantity_in_stock:
        raise ValidationError('Issued Quantity Exceeds Available Quantity')
    self.item.decrease_stock_quantity(self.issued_quantity)
    super(IssuedItem, self).save(*args, **kwargs)

• The if should be moved to clean().
• self.item.decrease_stock_quantity(self.issued_quantity) in the save feels very, VERY wrong! Not to mention a HUGE side-effect for something that shouldn't be happening.
• super(IssuedItem, self).save(*args, **kwargs) missing a return and remove IssuedItem, self from super().

While i'm at it, install and run ruff.

This is a very naive approach to the issue and it's a race condition away from being a day+ debugging nightmare for you.

def increase_stock_quantity(self, amount):
    self.quantity_in_stock += amount
    self.save()

def decrease_stock_quantity(self, amount):
    if amount > self.quantity_in_stock:
        raise ValueError('The Amount is Larger Than the Quantity in Stock')
    self.quantity_in_stock -= amount
    self.save()

1. use locks (.select_for_update())
2. (or) use query set self.objects.filter(id=self.id).update(quantity_in_stock=F('quantity_in_stock') + 1)

Also make sure you wrap this inside a transaction.

def save(self, *args, **kwargs):
    if self.issued_quantity > self.item.quantity_in_stock:
        raise ValidationError('Issued Quantity Exceeds Available Quantity')
    self.item.decrease_stock_quantity(self.issued_quantity)
    super(IssuedItem, self).save(*args, **kwargs)

• The if should be moved to clean().
• self.item.decrease_stock_quantity(self.issued_quantity) in the save feels very, VERY wrong! Not to mention a HUGE side-effect for something that shouldn't be happening.
• super(IssuedItem, self).save(*args, **kwargs) missing a return and remove IssuedItem, self from super().

While I'm at it, install and run ruff.