As a general rule it is the client that "owns" the attorney-client privilege, and has the right to waive it. The problem for you is that the privilege was not personal to you, but to the enterprise, the company you work for, and it was not the company's intention to waive the privilege. Hence, you took action outside the authority of your position, which could threaten your employment.
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This answer is offered as a public service for general information only and may not be relied upon as legal advice.
Like most legal situations there are two issues that need to be considered in your question. The first issue has been addressed in the previous answer. You are not the client but the government agency for which you work is the client. By sharing this you did not violate an attorney client privilege as you are neither the client nor the attorney. However, the government agency appears to have wanted the matter to be kept confidential. Thus, the second issue is - can you be fired for this leak of the memo/e-mail that the agency wanted to be kept confidential until it wanted to release the final memo? The answer to this question is far more complex. If we assume that you have been a great employee for many years, have had excellent performance reviews, you have not done anything recently to take up an adversarial relationship with your boss, the leak of the memo/e-mail has caused no significant consequences for the agency and the leak was truly accidental then I would say you have a very limited chance of being fired and if you were you would have an excellent chance of being reinstated upon appeal. However, if you have been having problems at work, have had a bad relationship with your boss, your performance has been problematic, the leak has caused serious implications or embarrassment for your agency/boss and it was an intentional or very careless act then your leaking of the memo could well be an offense for which termination is justified. You will need to evaluate your situation in a honest and realistic manner to determine where you stand.
This information is for discussion purposes only and does not constitute legal advice nor does it establish an attorney-client relationship. Individuals seeking a legal opinion or legal advice on any matter should contact an attorney experienced in the field of their issue and discuss retaining such attorney to represent the individual.