The moment you enter into hex editing these records is the moment that you are into trouble anyway - assuming the software is trusted in the first place. In so

The moment you enter into hex editing these records is the moment that you are into trouble anyway - assuming the software is trusted in the first place. In some countries it isn't really the POS that does the bookkeeping, but a special device is used that communicates with tax registry (or something, i'm not 100% sure on this). All the POS software does is tell the device what it was sold, the device registers it with the tax registry and gives back some sort of code string to be printed on the receipt. This also allows the receipt to be any type of paper (as opposed to the special triple-layered paper that was often used back in the day) and even if the device itself is tampered with, the information has already been sent to the tax registry. I think the author of this POS is mostly using this as an excuse to not release the source code, not for any practical reasons.