The Bonus Bet must be used in full. How are Bonus Bets paid out? When you place a bet using a Bonus Bet the winnings are calculated differently to a cash bet. A bet placed with a Bonus Bet will pay out the winnings minus the Bonus Bet stake.
The bonus money received as part of the 1Win offer can be spent for betting on sports. If there is not enough money in your main account, the bonus balance is used for betting.