Some casino's allow bonus buys and dont see that as breaching the terms. I know that at Casumo its allowed and i think Leovegas to.
All i can do is hope that they read the transcript and fix it, but have a hard feeling about it.
Seldom bonus buys are not allowed as per the terms, but for most bonuses maximum bet is defined in the (bonus) Terms & Conditions.
The maximum bet is often about EUR/GBP 5, which makes almost all bonus buys too big a bet.
Always read the T&C carefully.
Casumo:
Terms for Casumo Bonuses and Rewards
6.4. Unless otherwise stated (such as within a Welcome Offer), the maximum bet allowed when using Bonus Money is five Euro (€5) per spin or fifty Euro cents (€0.50) per bet line, or the approximate equivalent thereof in the currency of the territory in which You are resident, and this is applicable until such time as the Wagering Requirements have been met. We reserve the right of voiding bets and winnings resulting from bets of larger amounts.
Mr. Green:
7. PROMOTIONAL PLAY RESTRICTIONS
7.9. You are not allowed to place any wagers that exceed the maximum bonus bet size when using bonus money. The maximum bonus bet size is €5 / $5 / £5 / 50 SEK / 50 NOK (or currency equivalent) per bet/spin or equivalent, unless it is stated differently in the specific terms and conditions tailored to each bonus campaign. Please note that it is your responsibility to ensure that you are always aware of the maximum bonus bet size. For cases where wagers are deemed to have exceeded the maximum bonus bet size, an account may be reviewed and the bonus funds confiscated in the process.
(Also note that there is an extensive list of excluded games for bonus play on Mr. Green, but it is technically enforced you cannnot play them while playing with bonus)
Leovegas:
No maximum bet stated.
Please note that the signup bonus is construed a little bit differently from a standard match bonus.
Generally Leovegas reload bonuses are paid
after the wagering (reward) instead of upfront.
The good about this, is that the wagering is much lower than industry standard and also that there are no restricted games or max bet.
Regarding your case, clearly you seem to have broken the terms of Mr.Green's Promotional Play Restrictions 7.9
I hope for you that you'll meet some leniency because of the live support misinformation.