If you receive a deposit bonus or sign up bonus for €25, and you place the full amount on the same match as above, you will win €37.50. You will in other words keep the full amount, as the €25 bonus is credited to you.
Bonuses usually have roll over requirements, so you might have roll over (bet) x times the bonus before you can make a withdrawal. But the bonus amount of €25 is yours.
These bonuses can only be claimed when registering for the first time, if not specifically written in the terms. There are also reload bonuses, which can also be claimed by existing customers. These bonuses usually appear around big events such as the World Cup, a tennis Grand Slam etc.
See other bookmaker bonuses.