invoiceninja/app/Services/Invoice/UpdateBalance.php

<?php
/**
 * Invoice Ninja (https://invoiceninja.com).
 *
 * @link https://github.com/invoiceninja/invoiceninja source repository
 *
 * @copyright Copyright (c) 2021. Invoice Ninja LLC (https://invoiceninja.com)
 *
 * @license https://opensource.org/licenses/AAL
 */

namespace App\Services\Invoice;

use App\Models\Invoice;
use App\Services\AbstractService;

class UpdateBalance extends AbstractService
{
    public $invoice;

    public $balance_adjustment;

    public function __construct($invoice, $balance_adjustment)
    {
        $this->invoice = $invoice;
        $this->balance_adjustment = $balance_adjustment;
    }

    public function run()
    {
        if ($this->invoice->is_deleted) {
            return $this->invoice;
        }

        $this->invoice->balance += floatval($this->balance_adjustment);
        
        if ($this->invoice->balance == 0) {
            $this->invoice->status_id = Invoice::STATUS_PAID;
        }

        return $this->invoice;
    }
}