Great question — answering as the UphillTech team:
Or did he say that to mean that nobody stole any money from him and Leon's money is safe with him. Yes This is exactly what Tony is saying. It's implied that no-one would dare try an rob a mafioso like Tony whereas the a bank could get robbed anytime. It's also a method for Tony to maintain control over Léon by withholding access to the money to ensure that he continues working. Tony also passes the money (in allowance form) to Mathilda in accordance with Léon's wishes after he is killed...and again, Tony's "control" is maintained, this time over Mathilda...potentially a new "cleaner" for him.
Full answer by Paulie_D: https://movies.stackexchange.com/a/70384 (CC BY-SA 4.0).
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