I have checked the law enforcement and usa: government: law enforcement tags here, the FBI website, googled FBI promotion procedures, FBI human resources and checked wiki. The closest I could get was a news report of an agent being barred from consideration for promotion for 1 year after having to refuse a promotion due to problems with selling her condo.
So, I'm hoping someone here knows if the FBI specifically bars a person from consideration for promotions after refusing a promotion, and how variable the duration of that bar might be.