I wouldn't sell it for less than $20k. A software company is the prime market for the domain. One employee's salary for a software company is likely going to be $150k or more EVERY YEAR. How much do you think they would spend for their company name or flagship brand name?
If they are asking about your domain, TimeTravel is not brandable for their use and not comparable. I'd expect to find a sci-fi website at that url. The only competitor is the .com version of your domain and they have little chance of purchasing it as it's hosted by MarkMonitor, which is another reason I wouldn't go under $20k. No small fish is using MarkMonitor.
Look how many related sites there are and how frequently it is used in startups. In my opinion it is way too popular to let it go for such a low amount for a company already in business.
The buyer might be heytimetraveler(dot)com. If it's a viable business, they have the money to pay. If they say they don't have the funds, offer a lease. I sold a ".com" domain last year to a software company that just did scheduling for scuba diving excursions and as a 1 man operation they pay $550 per month for the domain with a $10k sales price. On top of that, they still use their ".co" domain and the .com just defaults to an error page. So a company that small, just bought it for brand protection.
To me, the revenue possibilities in scuba diving scheduling software as opposed to the many uses of "Time Traveller", are not comparable. I wouldn't budge off $20k and tell him you offer a lease plan, but you get a bit of a discount in fees for paying upfront. So without saying it you are already telling him that the price you quoted is the discounted amount, and you are eliminating their "budget" argument by offering lease payments.
The only thing that he can fall back on is the argument that the domain is not "worth" that much. Which is when you tell him your original plans for the domain and how you wanted to ensure your competitor didn't receive the better domain because your didn't want your customers to end up on the competitors website, or just as bad, end up on a site that had nothing to do with your service, causing additional confusion to the customer.