From my layman's understanding, this doesn't work. Just because you are building something as a work-for-hire doesn't immunize you from being sued, if you are violating someone else's patents in the process. You can have a contract with the buyer that says that the indemnify you in the event of a suit, but the way that works is that you still get sued and then you can go after the buyer to cover your costs when you lose or settle.

Again, not a lawyer, but this is my understanding.