This is at the bottom of single-claims.php, getting all data from an invoice

single-claims.php

<?php
    $invoice_fields = get_fields($fields['cla_fac_factuur'][0]);

    $inv_customer = array(
        'first_name' => $invoice_fields['inv_klant_voornaam'],
        'last_name' => $invoice_fields['inv_klant_achternaam'],
        'email' => $invoice_fields['inv_klant_email'],
        'company_name' => $invoice_fields['inv_klant_bedrijfsnaam'],
        'street_number' => $invoice_fields['inv_klant_straat_huisnummer'],
        'zipcode' => $invoice_fields['inv_klant_postcode'],
        'city' => $invoice_fields['inv_klant_woonplaats'],
        'country' => $invoice_fields['inv_klant_land']
    );

    $inv_invoice = array(
        'date' => $invoice_fields['inv_fac_datum'],
        'invoice' => $invoice_fields['inv_fac_factuurnummer']
    );
?>

<?php
    echo "<pre>";
    var_dump($fields);
    echo "</pre>";
    echo "<pre>";
    var_dump($invoice_fields);
    echo "</pre>";
?>

    <div class="container">
        <div class="row header">
            <div class="col-md-6">
                <h1><?php echo $inv_invoice['invoice']; ?></h1>
            </div><!-- /col-md-6 -->

            <div class="col-md-6">
                <div class="logo--wrapper">
                    <img class="logo" src="http://www.spatotaal.nl/wp-content/uploads/2014/08/cropped-spatotaal-header-logo.png" alt="Retro-Design logo">
                </div><!-- /logo--wrapper -->
            </div><!-- /col-md-6 -->
        </div><!-- /row -->

        <hr>

        <div class="row">
            <div class="col-md-6">
                <address>
            <?php
            if(!empty($inv_customer['company_name'])) {
                echo $inv_customer['company_name'] . '<br>';
            }

            if(!empty($inv_customer['first_name'])) {
                echo $inv_customer['first_name'] .' '. $inv_customer['last_name'] . '<br>';
            }

            if(!empty($inv_customer['street_number'])) {
                echo $inv_customer['street_number'] . '<br>';
            }

            if(!empty($inv_customer['zipcode'])) {
                echo $inv_customer['zipcode'] .', '. $inv_customer['city'] . '<br>';
            }

            if(!empty($inv_customer['country'])) {
                echo $inv_customer['country'];
            }
            ?>
                </address>
            </div><!-- /col-md-6 -->

            <div class="col-md-6 text-right">
                <address>
                    SpaTotaal<br>
                    Frans Erensstraat 2<br>
                    5921 VG, Venlo<br>
                    Nederland
                </address>

                <address>
                    Venlo, <?php echo $inv_invoice['date']; ?>
                </address>
            </div><!-- /col-md-6 -->
        </div><!-- /row -->

        <div class="row table-padding">
            <div class="panel panel-default">
                <div class="panel-heading">
                    <h3 class="panel-title">Factuur #<?php echo $inv_invoice['invoice']; ?></h3>
                </div><!-- /panel-heading -->

                <div class="panel-body">
                    <table class="table">
                        <thead>
                            <tr>
                                <td>Product</td>
                                <td class="text-right">Prijs exclusief BTW</td>
                            </tr>
                        </thead>
                        <tbody>

                <?php
                $btw = 0;
                $price = 0;

                for($i = 0; $i < count($invoice_fields['inv_pro_repeater']); $i++) :
                    $products = get_field('inv_pro_repeater', $invoice_fields['inv_pro_repeater'][$i]->ID);

                    var_dump($products);

                    $products = $products[$i];

                    $btw += $products['inv_pro_btw'];
                    $price += $products['inv_pro_prijs_inc'];

                ?>

                <tr>
                    <td><?php echo $products['inv_pro_onbekend']; ?></td>
                    <td class="text-right">&euro;<?php echo $products['inv_pro_prijs']; ?></td>
                </tr>
                <?php
                endfor;
                ?>
                <tr>
                <td class="text-right"><strong>BTW</strong></td>
                <td class="text-right">&euro;<?php echo $btw; ?></td>
                </tr>
                <tr>
                <td class="text-right"><strong>Totaal inclusief BTW</strong></td>
                <td class="text-right">&euro;<?php echo $price; ?></td>
                </tr>
                        </tbody>
                    </table><!-- /table -->
                </div><!-- /panel-body -->
            </div><!-- /panel panel-default -->
        </div><!-- /row -->

        <div class="row">
            <div class="col-md-12">
                <p>
                    Gelieve het volledige bedrag van <strong>&euro;<?php echo $price; ?></strong> binnen 30 dagen over te maken op rekeningnummer <strong>NL09INGB0684066750</strong> onder vermelding van kenmerk <strong><?php echo $inv_invoice['invoice']; ?></strong>.
                </p>
            </div><!-- /col-md-12 -->
        </div><!-- /row -->
    </div><!-- /container -->
</body>

First .... get the relationship field by doing the following.

$invoices = get_field('invoices');

Then, you'll want to loop through that (it'll be an array) by doing the following

foreach($invoices as $post):

Notice how I've used $post there, thats a wordpress thing that will automatically setup the post data for a wordpress loop. After the endforeach or whatever, you'll need to use the wp_reset_postdata().

Within the foreach loop, you can then get the values for the invoice you're currently on, like so:

$field1 = get_field('field1');