$query = "SELECT  SUM(amount) FROM income_daily";
    $sum = mysqli_fetch_row(mysqli_query($con, $query));
     echo "Total Income : ". "¢" .$sum[0];

     echo '<br>';
    echo '-';
    echo '<br>';

     $query1 = "SELECT  SUM(amount) FROM expenses_daily";
    $sum = mysqli_fetch_row(mysqli_query($con, $query));
    echo "Total Expenses : ". "¢" .$sum[0];

There are a few ways you could do this, depending on your needs. Here's a botch-job, but gives a single value based on a date:

SELECT (SELECT COALESCE(SUM(di.`amount`),0) FROM `daily_income` di WHERE di.`income_date` = ?) - (SELECT COALESCE(SUM(de.`amount`),0) FROM `daily_expenses` de WHERE de.`expenses_date` = ?) AS daily_difference

The question marks are anonymous placeholders for use with PDO or mysqli prepared statements - so you will need to bind these parameters (date). But if you want a nice one-liner with date data you have sanitized:

echo $mysqli->query("SELECT (SELECT COALESCE(SUM(di.`amount`),0) FROM `daily_income` di WHERE di.`income_date` = '$date') - (SELECT COALESCE(SUM(de.`amount`),0) FROM `daily_expenses` de WHERE de.`expenses_date` = '$date') AS daily_difference")->fetch_object()->daily_difference; 

Alternatively of course, you could just retrieve both table data sums individually and subtract using php!

BTW - the "coalesce" may be required as SUM gives NULL if no values exist for it - and therefore the difference will also be NULL, regardless of whether the other SUM value is not null.