Imho odds are your manager is right. Are you working for a 3 letter agency, its contractor or subcontractor; are you revealing trade secrets or company priorities, or implying them by revealing your knowledge on a subject; are you using your role as employee of company X to land paid speaking roles; have you signed employment agreements that state any intellectual property you create on or off the job is property of the company; are you speaking at event which could bring suspicion or disrepute to your employer eg blackhat, scammy, or events sponsored by foreign powers; do you work in finance and are being paid an amount in cash or in perks (transport, meals) which invites scrutiny for ethical violations; do you work in a sensitive or "access" position which has required you to sign a code of conduct? What's the reason for denial? Context is necessary.
> do you work in finance and are being paid an amount in cash or in perks (transport, meals) which invites scrutiny for ethical violations
I too would love to hear the reasons behind the manager's request, but wow, is this also a potential issue? Are you not allowed to be paid however much they are willing to pay you? And why would you disclose your salary or perks at a conference anyhow? Do people do that?
