invoiceninja/app/Services/Invoice/ApplyPayment.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\Services\Invoice;

use App\Models\Invoice;
use App\Models\Payment;
use App\Services\AbstractService;
use App\Services\Client\ClientService;

class ApplyPayment extends AbstractService
{

    private $invoice;

    private $payment;

    private $payment_amount;

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

  	public function run()
  	{
        $this->payment
             ->ledger()
             ->updatePaymentBalance($this->payment_amount*-1);

        $this->payment->client->service()->updateBalance($this->payment_amount*-1)->save();

        /* Update Pivot Record amount */
        $this->payment->invoices->each(function ($inv){
            if ($inv->id == $this->invoice->id) {
                $inv->pivot->amount = $this->payment_amount;
                $inv->pivot->save();
            }
        });

        if ($this->invoice->hasPartial()) {
        //is partial and amount is exactly the partial amount
            if ($this->invoice->partial == $this->payment_amount) {
                $this->invoice->service()->clearPartial()->setDueDate()->setStatus(Invoice::STATUS_PARTIAL)->updateBalance($this->payment_amount*-1);
            } elseif ($this->invoice->partial > 0 && $this->invoice->partial > $this->payment_amount) { //partial amount exists, but the amount is less than the partial amount
                $this->invoice->service()->updatePartial($this->payment_amount*-1)->updateBalance($this->payment_amount*-1);
            } elseif ($this->invoice->partial > 0 && $this->invoice->partial < $this->payment_amount) { //partial exists and the amount paid is GREATER than the partial amount
                $this->invoice->service()->clearPartial()->setDueDate()->setStatus(Invoice::STATUS_PARTIAL)->updateBalance($this->payment_amount*-1);
            }
        } elseif ($this->payment_amount == $this->invoice->balance) { //total invoice paid.
            $this->invoice->service()->clearPartial()->setStatus(Invoice::STATUS_PAID)->updateBalance($this->payment_amount*-1);
        } elseif($this->payment_amount < $this->invoice->balance) { //partial invoice payment made
            $this->invoice->service()->clearPartial()->setStatus(Invoice::STATUS_PARTIAL)->updateBalance($this->payment_amount*-1);
        }

        return $this->invoice;
  	}
}