akaunting/app/Providers/ValidationServiceProvider.php

<?php

namespace App\Providers;

use App\Models\Setting\Currency;
use Illuminate\Support\ServiceProvider;
use Validator;

class ValidationServiceProvider extends ServiceProvider
{
    /**
     * Bootstrap any application services.
     *
     * @return void
     */
    public function boot()
    {
        $currency_code = null;

        Validator::extend('currency', function ($attribute, $value, $parameters, $validator) use(&$currency_code) {
            $status = false;

            $currencies = Currency::enabled()->pluck('name', 'code')->toArray();

            if (array_key_exists($value, $currencies)) {
                $status = true;
            }

            $currency_code = $value;

            return $status;
        },
            trans('validation.custom.invalid_currency', ['attribute' => $currency_code])
        );

        $amount = null;

        Validator::extend('amount', function ($attribute, $value, $parameters, $validator) use (&$amount) {
            $status = false;

            if ($value > 0) {
                $status = true;
            }

            $amount = $value;

            return $status;
        },
            trans('validation.custom.invalid_amount', ['attribute' => $amount])
        );
    }

    /**
     * Register any application services.
     *
     * @return void
     */
    public function register()
    {
        //
    }
}