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Yes, a dismissed hero cannot be purchased on the day of their dismissal. If it weren't for this wise rule, everything would be much easier. But a hero who has fled can be purchased. However, you shouldn't use them. That is, you shouldn't use them at all; you can't even take them out of the Castle to dismiss them.
According to your reasoning, the deadline for Kyiv will be 22:59. But it's not that important – one hour doesn't change much, especially for me. The Tournament Walkthrough stopped on the sixth day. I'll finish it later.
Thank you to the organizers for the map and the Tournament.