united states - How do professors choose their summer salary?

In the United States, most professors are paid by the university on a 9 month basis for teaching (Sep - May). During the summer months a professor is typically paid from their grants and other revenue sources. How is the summer salary chosen? E.g. if a professor is paid 10kUSD/month, can (s)he choose to be paid 20kUSD/month during the summer?