Katie Furlong, 23, who worked as a debt officer at the taxpayer-owned bank, had been due to receive the payment as she was one of 3,500 people who had been told they would lose their jobs.
The bank had not returned calls for comment at the time of writing.
Having heard about the redundancies, Furlong went on Facebook and posted: "WoOOOOooooOooooHOoooOooOoo' it was pretty damn obvious something like this was coming. I'm neither stupid nor naive...quote honestly it is the best news ever as far as I am concerned!"
Later she added: "They will give us the option to take early retirement (for those eligible obviously), transfer to Birmingham and if so, the possibility of a travel allowance, or redundancies. Either way, SCORE!!!"
Furlong also wrote to a friend on Facebook, saying: "I've just hung on by my fingertips to stick around long enough for a nice payout when they could've had me out long ago without a penny! More fool them!"
RBS - which became aware of the messages after a colleague of Furlong's informed managers - later sacked Furlong, claiming she had breached its "declaration of secrecy" and that the Facebook messages amounted to gross misconduct. They stopped the £6,000 payment.
Furlong, who had worked at the bank in Telford for over three years, has said she will take RBS to an employment tribunal, claiming unfair dismissal.
"I can't believe I've been treated so appallingly for what essentially amounts to having a chat with my mates outside of work," she said.