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No, it wouldn't be a breach of the lease for the landlord to waive his right to prohibit subleasing. He has the right to prohibit it, but not the obligation. So he can allow it if he chooses, or prohibit it.
As for releasing you from the lease, if you have something in writing, then that would be legally binding, but if this was just an oral statement, that is hard to prove if it got to court in a breach of contract action.