
Obviously, you are not entitled to any severance payment. New employment has not started and, thus, there is no termination of employment. When there is no dismissal, there is no case for an unfair dismissal.
You may not be able to prove there was a collaboration between your ex-employer and your would-be new employer to get you out of the job without paying anything.
But check the employment offer you have accepted and countersigned.
There may be a clause stating that the contract may be terminated by either party serving a written advance notice not shorter than two to three months.
Then sue the company for breach of contract, calling for compensation for damage.
There is a chance the Labour Court will order the payment equal to salary in lieu of the advance-notice period.