如何在 Laravel5 请求类中验证 Money

我的验证类如下所示

<?php

namespace App\Http\Requests;

use App\Http\Requests\Request;

class PaymentRequest extends Request
{
    /**
     * Determine if the user is authorized to make this request.
     *
     * @return bool
     */
    public function authorize()
    {
        return true;
    }

    /**
     * Get the validation rules that apply to the request.
     *
     * @return array
     */
    public function rules()
    {
        $rules = array(

                       'invoiceid'=>'required',
                       'recieved_amount'=>'required',
                       'ref_no'=>'required',
                       'date'=>'required',
                       'comment'=>'required'
                       );


    }
}

我想验证为“钱”字段 如果输入了除钱以外的任何内容，则应进行验证recieved_amount

任何人都可以在这方面帮助我吗？