I received the welcome sports bonus, wagered it, had a 100% bar (I also carefully calculated the stakes), and played a few more bets, the last two of which turned out to be winnings (very large amounts for me: 4444 and 1800). Bookmaker cancelled these winnings, leaving me with 1700 PLN, which is the maximum bonus payout (I deposited 850 PLN, received 850 PLN). Their terms and conditions state that the maximum bonus payout is 1700 PLN. The problem is that I had already wagered the bonus well beforehand, and Bookmaker's explanation is that "All bets placed during the bonus period are considered to have been placed during the wagering process and are limited to the maximum bonus payout amount when the funds are transferred from the bonus balance to the real balance."
I read the bonus terms and conditions carefully, and there's nothing to that effect.
