Textbook publisher offers to pay me for a comparative review of textbooks I'm currently teaching a fairly large introductory class,
with about 500 students per semester (spread across multiple sections).

Recently, I received an e-mail from McGraw-Hill Education
offering me a few hundred USD to complete a comparative review
of our current textbook (also McGraw-Hill)
and another McGraw-Hill textbook.

I am not sure what to make of this.
On the one hand,
it definitely sounds nice to me
to spend an hour or two to look through a new textbook,
and write a comparative review,
and be somewhat well compensated for my time.
On the other hand,
it feels like they are trying to "bribe" me
to adopt another textbook,
which may perhaps be more profitable for them
than our current recommended textbook.

Should I be wary about taking the money and accepting this offer?
Is there something unethical about this?
Also, would my university be upset about such an arrangement?
I suspect this would happen at most once a year,
if it indeed happens in the future.
I see nothing fundamentally unethical about this. It would become unethical if the request included a requirement that you adopt a particular book, or that you write something that pleases them. But by itself, requesting a review and paying you for it does not seem wrong to me -- they may simply be interested to hear from people in the trenches how these two books compare.