Civil service offer usually can negotiate or not if not meeting expected salary?

Is it a take-it-or-leave-it situation since the offered salary amount is approved by management and is derived according to civil service HR guidelines and policies, and unlikely to be revisable?

For example, in resume, we state amount $X as expected salary as instructed in the job listing. $X is a conservative 50% percentile market rate for a professional in certain industry and number of years of experience.

Then an offer below $X comes for a civil service position. Maybe HR trying to drive a hard deal to save money for govt. Then how? Request to negotiate or give an answer on-the-spot (accept/reject)?