An employer can make you pay for the spoiled product if it is your responsibility to keep products on the cooler. However, you should check your contract to see if there is anything that can exempt you from paying. In addition, you should also check if there is anything in the employment standards law which covers that issue.
Alls he has to do is send it back to the company he got it from and they will replace it. For him to take it out of your pay I would think its illegal, but since the problem is so easily rectifiable he shouldn't have to go to that extreme.