I am currently employed by a company whose core business is not IT-related but I have developed a few applications that are central to some crucial business processes and I want to create a startup around them.
I have therefore anounced my intention to the boss, explaining that I planned to have him as first client to continue to maintain and improve those apps. The deal I proposed was that they transfer the intellectual property of those applications to me and I will give them preferential terms as first client + make sure the transition is as smooth as possible.
Although he agreed in principle at first, he now wants to make the IP transfer conditional on a scheme (to be defined) that would show his clients that I am still committed to the company because he considers that I am a key person and wants to make sure his clients are not under the impression that I am gone.
The existing clients are a red herring here. Whether you are an employee, contractor or provider, as long as the work gets done, they shouldn't need to know the nature of the relationship between you and your current boss.
The remaining answer is simple: Draw up a contract between your current boss and your new business, detailing what needs to be done against what payments, and stipulate the agreed IP transfer.
That said - a delayed IP transfer could be a recipe for disaster - what happens if the contract is breached before the IP transfers, or or or or. Seek a lawyer.