Incrementally add or substract from usermeta field

The function below should do your job.

function manage_credits( $operation = 'add', $amount = 0 ) {
    $user_id = get_current_user_id();

    if ( $user_id ) {
        $user_credit = (int) get_user_meta( $user_id, 'my_user_credit', true );

        if ( 'add' == $operation ) {
            $user_credit += $amount;     //For adding credits
        } elseif ( 'remove' == $operation && ( $user_credit >= $amount ) ) {
            $user_credit -= $amount;     //For removing credits
        } elseif ( 'remove' == $operation ) {
            echo 'You do not have enough credits to post.';
            return;
        }
        update_user_meta( $user_id, 'my_user_credit', $user_credit );
    }
}

Now all you need to do is while triggering add you will call manage_credits( 'add', 10 ) and while triggering remove you will call manage_credits( 'remove', 10 ). The second parameter should be passed with the amount of credit you would like to add or remove.