i'm thinking of making sandwiches at home and bringing them to the office. Is that okay? I mean, does tonkatsu sandwich have to be hot or warm before it should be eaten? I'm thinking of selling this at our office.
As long as the tonkatsu is cooked and then cooled appropriately (ie relatively quickly), then stored at appropriate temperatures (ie refrigerated) until consumption, you should be fine. The length of your journey to work will be the main factor - you need to keep the food cool enough, so a cool bag with ice blocks may be in order. Then you need to make sure they remain cool until they are consumed. Whether or not they should be reheated is probably a matter of taste.
Also, check your local regulations for selling food for public consumption. It will vary from country to country, but many places will require a kitchen inspection and permit before you can legally sell food.