Client wants to buy an individual file from a group photo shoot? What do?

I've got a bit of a weird question that honestly, even after quite a bit of searching, I haven't seen many people discuss as I think it's sort of an ethics question.

A few weeks ago I did headshots for a company of about eight total people. The rights to the photos, as far as the contract is concerned, stayed with me (the photographer), and the only other party mentioned was the owner of the company I was employed by.

Now, however, I have someone from the company approaching me about purchasing an individual headshot from the group of photos. He's seen the shot and liked it, but since the contract was between myself and the owner, he wants to purchase the use of the image from me.

I'm not quite sure what to do - should I go ahead and give him permission to use the file? Or should I charge him some nominal fee for rights to use the image? This isn't really spelled out in the contract, and it seems like a bit of a weird scenario, so any advice would be appreciated.

edit: Ok - it seems like people are down to sell the image, which I'm 100% ok with. The next question I guess being what an ethical amount to charge would be? The total charge to the customer was $325 for headshots for 8 people, and around 2 hours of shoot time. Should I do it as a percentage of the total amount charged? It seems a bit strange to charge my full individual headshot photo shoot amount for one photo...

edit 2: Thanks everyone for the input! Since it's a local company, and someone I feel I'll be working with in the future, I told him it was no trouble to send over the file, as long as he cleared it with the original client first. I also offered to have some physical copies printed and delivered if he'd like, for a small fee for labor/time/etc. I really like the discussion tho because, as I said above, I feel like this is more of an ethical choice (building a good relationship with client (possibly) vs. knowing when and what to charge for) and I always find those discussions interesting, especially in an industry like photography that's often at the whim of word-of-mouth, image, etc.