Are you an Ambassador Elite member? Rumour has it that an Ambassador has the scope to "lend" points to an Ambassador Elite member (to be later reclaimed when earned).

That said, I'm not sure you can buy 300,000 points, the limit appears to be 250,000, so you might actually be 80k short.

"In 2025, members may purchase or receive as a gift a combined maximum of 150,000 Points per calendar year when purchased through our Buy Points and Gift Points pages (this website), Buy Points Points.com-powered window on Marriott websites, Marriott Mobile Apps or the Customer Engagement Centers. [...] Members may purchase up to an additional 100,000 Points annually while booking a hotel stay through the reservation path on marriott.com."