If possible, bets are always paid for with your withdrawable balance. This comes with the advantage that any profit made with these bets are becoming part of your withdrawable balance as well.
Only when your withdrawable balance does not have enough funds to pay the total stake of a bet slip we will deduct the remainder from your bonus balance. Any returns will then be credited pro rata to the respective balances. Please note that irrespective from which balance the stake is taken it always counts towards the turnover requirement of your currently active bonus.