This is possible, many merchants are already using services like Groupon and that same approach would suit you here.



The first thing to do would be to set the product up with its regular price, and then create a long series of one-use discount codes for the full price of the product using our discount feature:

http://www.e-junkie.com/ej/help.discounts.htm



Next you just need to find a way to distribute one code each to everyone who is paying you outside the cart. If you are taking payments in person you could give each buyer a business card with the code printed on it, or if you actually are using Groupon itself or a similar service they are already set to distribute codes and probably can give a list to be entered for your discounts in the first place.



That way when someone pays you otherwise they simply go to your site, add the product to the cart and then enter the discount code to waive the price and checkout for free.



Basic E-junkie accounts only allow up to 100 free checkouts a day, but if your expected traffic falls in that range already you've got nothing to worry about.