I'd recommend a "rain check", suggesting that as soon that specific product is replenished, you'll reserve one for them so this wouldn't happen again. I would also apologize for not having been prepared, and offer a small coupon or two as a form of compensation for the inconvenience. (That is, if you're able to offer coupons.) Another thing that would be important is to display professionalism in a stressful situation. That will show your higher ups that you are an asset to the business. Hope I helped! :)