Elon Musk‘s aerospace company raised a whopping $75 billion by selling 555.6 million shares, or about 5% of its stock, in Friday’s much-awaited market debut. However, in the weeks ahead, SpaceX could fetch billions more!A mechanism, known as a greenshoe option, gives Morgan Stanley, the stabilisation agent for the listing, the right to buy additional SpaceX shares at the IPO price of $135 apiece for up to 30 days after trading begins. If fully exercised, the bank could acquire around 83 million more shares from the company.That would take the total number of shares sold to 638.9 million and increase the amount raised by a further $11.2 billion, according to a Reuters report.Unlike the shares already offered to investors, these additional shares have not yet been issued by SpaceX. Instead, the process requires underwriters to first sell the stock through a short position in the market. They can then either purchase the shares from SpaceX at the original offer price or buy them back from the open market, depending on how the stock performs.The route taken largely depends on where the share price moves after the listing.If a stock trades above its IPO price, underwriters can exercise the greenshoe option and purchase the extra shares directly from the company, enabling the issuer to raise additional capital while covering their short position. Strong investor demand can therefore translate into a larger fundraising haul for the newly listed firm, Reuters reported.If the stock falls below the offer price, underwriters are more likely to buy shares in the open market instead. This allows them to close out their short position while helping to support the stock and reduce volatility during the early stages of trading.The greenshoe, formally called an over-allotment option, has long been used in major public offerings as a tool to promote more orderly trading. It traces its origins to Green Shoe Manufacturing, which became the first company to employ the structure during its 1960 IPO.Its use has shaped some of the world’s most prominent listings.In Alibaba’s 2014 debut, soaring demand pushed the stock 38% above its $68 offer price on the first day of trading. Underwriters subsequently exercised the entire 15% greenshoe option, purchasing an additional 48 million shares from the company at the IPO price. The move boosted total proceeds from the listing to around $25 billion, making it the largest IPO on record at that time.Uber’s experience five years later unfolded differently. After pricing its shares at $45, the ride-hailing company’s stock slipped below that level amid concerns over its route to profitability and wider market weakness. Rather than buying additional shares from Uber, underwriters stepped into the market to purchase stock directly, aiming to ease selling pressure. Even so, Uber’s shares ended their first trading session 7% lower.For SpaceX, the next few weeks will determine whether its landmark IPO remains a $75 billion fundraising event or grows even larger through the exercise of the greenshoe option.