Hi,

1.Create Access Rights in XML

   id,name,model_id:id,group_id:id,perm_read,perm_write,perm_create,perm_unlink  access_credit_limit_admin,access_credit_limit_admin,model_credit_limit,base.group_system,1,1,1,1

     access_credit_limit_user,access_credit_limit_user,model_credit_limit,,1,0,0,0

2. Modify the Model

class CreditLimit(models.Model):

_name = 'credit.limit'

_description = 'Credit Limit'

name = fields.Char(string='Credit Limit Name')

credit_limit = fields.Float(string='Credit Limit Amount')

approved = fields.Boolean(string='Approved', default=False)

  @api.multi def action_approve(self):

         if not self.env.user.has_group('base.group_system'):

             raise AccessError("You do not have permission to approve credit limits.")

              self.approved = True

3. Update the View

<odoo>

    <record id="view_credit_limit_form" model="ir.ui.view">

        <field name="name">credit.limit.form</field>

        <field name="model">credit.limit</field>

        <field name="arch" type="xml">

            <form>

                <sheet>

                    <group>

                        <field name="name"/>

                        <field name="credit_limit"/>

                    </group>

                    <footer>

                        <button name="action_approve"

                                type="object"

                                string="Approve"

                                states="draft"

                                attrs="{'invisible': [('approved', '=', True)]}"

                                class="btn-primary"/>

                        <button string="Cancel" class="btn-secondary" type="object" name="action_cancel"/>

                    </footer>

                </sheet>

            </form>

        </field>

    </record>

    <record id="view_credit_limit_tree" model="ir.ui.view">

        <field name="name">credit.limit.tree</field>

        <field name="model">credit.limit</field>

        <field name="arch" type="xml">

            <tree>

                <field name="name"/>

                <field name="credit_limit"/>

                <field name="approved"/>

            </tree>

        </field>

    </record>

</odoo>

Hope it helps