Nicky in a voice over makes it clear that no other mobsters will rob the casino and he talks very obliquely but significantly to the two men at the cage who then leave hurriedly without completing some paperwork which Nicky tells the cashier they "don't need anymore."
Clearly these two men understood they should rapidly stop what they were planning to do and they seem grateful that they were allowed to walk away. But just what was it that Nicky understood they were trying to do? He mentions 200 grand. Was it perhaps simply that they planned to get credit and then cash in their chips? It is not easy to steal from a casino in this way or everyone would do it.