The following sentence has been divided into four segments. Identify the segment that has a grammatical error.
When he resigned, / the company offered him / a huge sum of money but / he refused to agree to it.

The segment that contains a grammatical error is: he refused to agree to it.

Explanation:
In English grammar, the usage of "agree to it" in the context of receiving an offer of money or compensation is incorrect.
We use "agree to" when giving consent to a request, plan, proposal, or set of terms. However, when dealing with an offer of money, gift, or job, the appropriate verb is accept (or to refuse to accept).
Therefore, the verb phrase "refused to agree to it" should be replaced with "refused to accept it" or simply "refused it".

Corrected Sentence:
"When he resigned, the company offered him a huge sum of money but he refused to accept it."