Hi Thierry,
The reward product would typically be marked as such in the master data and be contained within a reward catalog. This is common with a lot of credit card companies, offering gifts to choose from using your accumulated points for example. In some cases, you need this to be flexible so that you can pay for something, e.g. a flight, with your points and top off the amount with fiat currency if you fall short.
In your case, you want to enable a flexible redemption for your general products sold within your business in the normal catalog.
Hope this helps,
Jehan