如何在PHP编码器中从字段中减去一个值

我有一个表分销商和用户。现在，每个用户都由他的经销商映射为一个名为KEY的字段，该字段唯一地定义了经销商!现在我有一个编辑用户的表单。在这里，如果经销商将用户的余额设置为yes，那么我想从经销商的余额字段中减去5欧元。下面是我的代码:

控制器检查经销商是否将余额设置为yes:

 if($this->input->post('balance') === "Yes")
         {
            $this->reseller_m->deduct();
         }

Mode函数，该函数应该从reseller balance中减去5:

public function deduct($balance)
{
    $this->db->set('balance', 'balance-5');
    $this->db->where('id' , $id);
    $this->db->update('reseller',$data);
}

public function edit ($id = NULL)
{
     $usertype=$this->session->userdata('usertype');
    if($usertype ==="reseller")
    {
    // Fetch a user or set a new one
    if ($id) {
        $this->data['user'] = $this->user_m->get($id);
        count($this->data['user']) || $this->data['errors'][] = 'User could not be found';
    }
    else {
        $this->data['user'] = $this->user_m->get_new();
    }
    // Set up the form
    $rules = $this->user_m->rules_admin;
    $id || $rules['password']['rules'] .= '|required';
    $this->form_validation->set_rules($rules);
    // Process the form
    if ($this->form_validation->run() == TRUE) {
        $data = $this->user_m->array_from_post(array('sip_id','sip_pass','name','email', 'password','phone','status','created','balance'));
        $data['password'] = $this->user_m->hash($data['password']);

        $this->user_m->save($data, $id);

         if($this->input->post('balance') === "Yes")
         {
            $this->reseller_m->deduct();
            //echo "goimng";
         }

        redirect('reseller/user');
    }
    // Load the view
    $this->data['subview'] = 'reseller/user/edit';
    $this->load->view('reseller/_layout_main', $this->data);

}
else{
    $this->load->view('permission');
}
}

帮我从转售商减去5只有当他编辑自己的用户，不应该影响别人。