i tried to get the last value of "Prestaciones Acumuladas" but i don't understand how it works, i'm using

@api.one

def _get_sum_payment(self):

payments = self.env[hr.prestaciones'].search([('employee_id','=', self.employee_id),('acumulado')])

self.sum_payments = sum(item.amount for item in payments)

sum_payments = fields.Float(string="Prestaciones Acumuladas", compute='_get_sum_payment', store="True")

To do that you need to have the field "Prestaciones Acumuladas" as a calculated field with store = True and also readonly to do not modify it latter at least that you made a change in the field "Abono Mensual" of previous saved values. The formula to calculate it's simply the sum of the field "Abono Mensual" + the field "Prestaciones Acumuladas" of the previous record. You just need to figure out how to get the previous record, could be based on the field create_date like a search over the model with a create_date minor than the current record create_date with a limit 1 of results and ordered by create_date DESC