How to "Open Source" Custom, Contract Software?

customWorks asks: "I've been approached to write a piece of custom software for a small business owner with the promise of autonomy in its design and implementation. I do not intend to stick around for incremental development after I've delivered it, and so I feel strongly that open sourcing the software would be prudent for the both myself and prospective client. That said, I still expect to be paid for the developing the software. The issue of course is over convincing the client of the benefit of giving away the source to something they've just paid to have developed. I'd like to know if any of you who've done similar contract work have had experience (success?) in presenting an argument for open sourcing the end product? What were the major concerns/misperceptions that you had to overcome in making the case for open source?"