overtime

Cannot answer this without further info. If under a Belgian contract overtime could be paid at the normal hourly rate but it could also be paid at a higher rate, often as much as double time. It is also often the case that companies will not pay extra for overtime as it can be considered 'obligatoire' where they may give an extra days leave etc. You need to read your contract if you have one

If he want's to give it cash, then 9 out of 10 it's not official, so no rules apply.