[robotresearcher]

'no possible summer salary'

Summer salaries are usually paid for by research funding raised by the professor from outside sources. Typically you make sure your students are supported, then travel and operating costs for your group, then if any funding is left over, you can pay yourself summer salary.

Is the university preventing him from raising funds? If he is not raising funds, then the expenses above could be unmet. It's hard to get tenure without covering your costs.

[derbOac]

Where I was at, in an entirely different field, it wasn't uncommon for faculty to obtain summer salary through summer teaching. Usually that was reserved for graduate students, but it was a normal thing for some faculty to have.

[robotresearcher]

backgrounder:

It's not obvious to outsiders that being a successful researcher is expensive, and the university does not pay for it after a short startup period. Harvard is unusual in that they have much more internal student scholarship money than most, which helps a lot. That subsidy, and the Harvard brand, means that Harvard faculty have a good advantage over the field when raising money.

On the other hand, a lot is expected of them. The university is considering the opportunity cost, and the opportunity is very valuable.