invoiceninja/app/Http/ValidationRules/PaymentAmountsBalanceRule.php

<?php
/**
 * Invoice Ninja (https://invoiceninja.com)
 *
 * @link https://github.com/invoiceninja/invoiceninja source repository
 *
 * @copyright Copyright (c) 2020. Invoice Ninja LLC (https://invoiceninja.com)
 *
 * @license https://opensource.org/licenses/AAL
 */

namespace App\Http\ValidationRules;

use App\Libraries\MultiDB;
use App\Models\User;
use Illuminate\Contracts\Validation\Rule;

/**
 * Class NewUniqueUserRule
 * @package App\Http\ValidationRules
 */
class PaymentAmountsBalanceRule implements Rule
{

    /**
     * @param string $attribute
     * @param mixed $value
     * @return bool
     */
    public function passes($attribute, $value)
    { 
        return $this->calculateAmounts(); 
    }

    /**
     * @return string
     */
    public function message()
    {
        return 'Amounts do not balance correctly.';
    }

    private function calculateAmounts() :bool
    {
        $data = [];
        $payment_amounts = 0;
        $invoice_amounts = 0;

        $payment_amounts += request()->input('amount');

        if(request()->input('credits') && is_array(request()->input('credits')))
        {
            foreach(request()->input('credits') as $credit)
            {
                $payment_amounts += $credit['amount'];
            }
        }

        if(request()->input('invoices') && is_array(request()->input('invoices')))
        {
            foreach(request()->input('invoices') as $invoice)
            {
                $invoice_amounts =+ $invoice['amount'];
            }
        }
        else
            return true; // if no invoices are present, then this is an unapplied payment, let this pass validation!

        return  $payment_amounts >= $invoice_amounts;

    }
}