If the underwriter has entered into a firm commitment to sell 1 million shares at $20 each, including a $2 spread, then the issuing firm will receive:
$20 - $2 = $18 per share
If only 500,000 shares are sold, then the issuing firm will receive:
$18 x 500,000 = $9,000,000
Therefore, if only 500,000 shares are sold, the issuing firm will receive $9,000,000.