Sync a custom product attribute with an invoice item

functions.php

<?php

add_filter( 'storeabill_woo_order_item_line_item_sync_props', 'my_child_sync_line_item_props', 10, 3 );

/**
 * @param $props
 * @param \Vendidero\StoreaBill\WooCommerce\OrderItemProduct $item
 * @param $args
 *
 * @return mixed
 */
function my_child_sync_line_item_props( $props, $item, $args ) {
	if ( $order_item = $item->get_order_item() ) {
		if ( $product = $order_item->get_product() ) {
			$attribute_key   = 'marke';
			$attribute_label = 'Marke';

			if ( $attribute_value = $product->get_attribute( $attribute_key ) ) {
				$props['attributes'][] = new \Vendidero\StoreaBill\Document\Attribute( array(
					'key'   => $attribute_key,
					'value' => $attribute_value,
					'label' => $attribute_label
				) );
			} elseif ( $product->get_parent_id() > 0 ) {
				/*
				 * In case the product is a child product - lets check the parent product
				 * for the attribute data in case it was not found for the child.
				 */
				if ( $parent = wc_get_product( $product->get_parent_id() ) ) {
					if ( $attribute_value = $parent->get_attribute( $attribute_key ) ) {
						$props['attributes'][] = new \Vendidero\StoreaBill\Document\Attribute( array(
							'key'   => $attribute_key,
							'value' => $attribute_value,
							'label' => $attribute_label
						) );
					}
				}
			}
		}
	}

	return $props;
}