How to write to variable get_template_file() ?

Hello everyone.
I need to make something unusual.

Everyone knows funtion get_template_part();
So, what i’m trying to do is:
I have a function that accepts few arguments, then makes a custom wp_query and loops throught posts (and with changing variables must change content).
So what i’m trying to do now is make get_template_part() not to echo the content inside but return it to my variable.
Quite ununderstandible…

may be code will help you to get my idea:
Somewere in file (not in loop)

$contents = unit_get_posts(array('post_type'=>'services','file_url'=>'services'));
    echo '<pre>';
    var_dump($contents);
    echo '</pre>';

Function in functions.php:

function unit_get_posts($args = array() ){
    $defaults = array (
        'file_pre'      => 'ajax',
        'file_url'      => 'news',
        'post_type'     => 'post',
        'posts_per_page'=> 10,
        'offset'        => 0,
        'echo'          => false
    );

    $args = apply_filters( 'unit_get_posts', $args );

    extract( wp_parse_args($args, $defaults), EXTR_SKIP );

    $return = '';

    $posts = new WP_Query(array(
        'post_type' => $post_type,
        'posts_per_page' => $posts_per_page,
        'offset' => $offset
    ));

    if($posts->have_posts()):
        while($posts->have_posts()): $posts->the_post();
            $return .= load_template_part($file_pre, $file_url);
        endwhile;
        wp_reset_query();
    else:
        $return = false;
    endif;

    if( $echo ){
        if( empty( $return ) ){
            echo 0;
        } else {
            echo $return;
        }
    } else {
        return $return;
    }
    die();
}

function load_template_part($template_name, $part_name=null) {
    ob_start();
    get_template_part($template_name, $part_name);
    $var = ob_get_contents();
    ob_end_clean();
    return $var;
}

But how hard i try my variable $return (on line $return .= load_template_part($file_pre, $file_url); ) doesn’t gets any value..

I just don’t know how to make return of echo..
how to make something like these

function first(){ echo '123'; }
function two(){
$text = first();
$text = str_replace('1','2',$text);
return $text;
}
two();

If someone knows how to do that PLESE tell me!!