Would you be happy with the bonus that they're offering you to give up the project completely and walk away from the project AND a job.

There's always the possibility that you don't like working for these people or it doesn't work out.

Also, I think you should talk to an accountant/lawyer about whether you want to be paid for the product as a bonus or as a separate sale for the IP. If they want to pay you for it as a bonus, there may be seriously negative tax implications for you...but if you sell it you want to make sure to shield yourself from liability.