Inherit the payslip model and add the field (add also to the form view)

class HrPyslip(models.Model):
    _inherit = 'hr.payslip'

    bonus_extra = fields.Float()

And use in your custome salary rule

<record id="my_custom_rule" model="hr.salary.rule">
    <field name="name">Custom_Rule</field>
    <field name="sequence">100</field>
    <field name="code">100</field>
    <field name="category_id" ref="hr_payroll.BASIC"/>
    <field name="condition_select">python</field>
    <field name="condition_python">result = bool(payslip.bonus_extra > 100)</field>
    <field name="amount_select">code</field>
    <field name="amount_python_compute">result=(payslip.bonus_extra*3.14)</field> 
</record>