i want to add value to a field by calculating two values for tow fields in django api rest framework not template

Question

#models file

I want to add value to deserved_amount field by calculating a payments minus from the class product inside field selling_price ... If there are any modifications to the important classes, I want to deduct a payment from the original amount and then show the remaining amount after each payment to the user... I want the process to rest framework api endpoint

# this class adds payment to each product price 

class Payment(models.Model):
  PK = models.AutoField(primary_key=True)
  payments =        models.FloatField(default=0)
  description_paid =    models.TextField(max_length=1500, 
  default='')
  payment_date  =  models.DateField(default=datetime.date.today)
  product = 
  models.ForeignKey('Product',related_name='paymentProducts' , 
  on_delete=models.CASCADE)
  imag_file = models.ImageField(upload_to='uploads' , blank=True , 
  null=True)
  deserved_amount = models.FloatField(default=0, null=True , 
  blank=True)
  '''
    I want to add value to deserved_amount field by calculating a 
    payments minus from 
    the class product inside field selling_price ...
    please help me , thanks
  '''



#this is class add a product this is class add a product this is 
#class add a product this is class add a product this is class add  
#**strong text**product 
class Product(models.Model):
  JAWWAL = 'JAWWAL'
  type_category = [
    (JAWWAL,'جوال'),
    ('SCREEN','شاشة'),
    ('FRIDGE','ثلاجة'),
    ('LAPTOP','لابتوب'),
    ('WASHER','غسالة'),
    ('ELECTRICAL DEVICES','جهاز كهربائي'),
    ('FURNITURE','موبيليا'),
    ('ATHER','اخرى'),
  ]
  PK =                models.AutoField(primary_key=True)
  cost_price =        models.FloatField(default=0)
  selling_price =     models.FloatField(default=0)
  supplier_name =     models.CharField(max_length=70 , blank=True ,  
  null=True)
  category =          models.CharField(max_length=19, 
  choices=type_category,default=JAWWAL)
  description =       models.TextField(max_length=1500, default='')
  date =              models.DateTimeField(auto_now_add=True , 
  blank=True ,  null=True )
  customer =          models.ForeignKey('customer', 
  related_name='customerProducts', on_delete=models.CASCADE)
  payment_deu_monthly = models.FloatField(default=0)