According to one laid off federal employee,

[Wiz Imp]

31. If they were rehired into the same position shortly after being let go...

https://www.opm.gov/job-seekers/reinstatement
I'm pretty sure, they would return at the same pay level and seniority level, etc. as when they left.

There was actually a new rule set by OPM recently allowing the Fed Government to rehire you at a higher rate of pay. I'm sure that doesn't really apply here.

https://www.fedmanager.com/news/opm-releases-new-rule-former-feds-can-be-re-hired-at-a-higher-pay-scale

OPM Releases New Rule: Former Feds Can Be Re-hired at a Higher Pay Scale

A final rule released by the Office of Personnel Management (OPM) on June 8, 2021 would make it easier for federal agencies to bring back former employees at a higher pay scale. Prior to the release of this rule, agencies could bring back ex-employees, but at the same pay grade they were receiving before they left federal service.

OPM recognizes the tendency of younger workers to move more freely than previous generations from one job to another. Additionally, the skills these workers can gain from other jobs in the private sector or academia could be useful when they return to work for government.

OPM wrote in the final rule, “Facilitating the return to Government of people who have broadened their work experience in this way advances the civil service’s goal of an effective and efficient government. Apart from providing the agency with additional choices in making selections for current vacancies, it enables agencies to build a workforce of individuals who bring a variety of knowledge, training and experiences to their work.”

Former federal workers who spent three years or less in federal service have a three-year window to return to their jobs. Those who worked for the government longer than three years have no limitations on when they can be rehired. In order to be considered, former federal workers must have been given a fully successful performance rating in their final year of service before leaving government.

https://www.opm.gov/job-seekers/reinstatement