Here is the original item:
Professor Neal loves chocolate cake.
(A) But could
not accept the slice that her student Michael
offered (B)
because she knew that it was a
bribe (C)
for a better grade on his midterm essay.
You wanted to do this:
Professor Neal loves chocolate cake. But could not accept the slice that
her student Michael offered because she knew that it was a
bribe,
for a better grade on his midterm essay.
A comma after bribe is an error. For a better grade on his midterm essay is a string of prepositional phrases. When you attach such phrases at the end of a sentence, you do not need punctuation.
You might want to review the rules.