Firing an employee for having too much money in the till is just.
It means he wasn’t minimizing the risk to himself or his fellow employees. He was also creating an environment where he increased the risk of a follow-up robbery by the robber or one of his co-patriots, because their robbery was rewarded.
Now, imo, it also depends how much he had in the register. If it was only $25, than the boss should have given him a warning, but if it was over $200 above float, as the store claimed than it means the clerk was not paying attention to what was in the till at all.
He's better off looking for a better job.