My guess is that you were already above 250kts when you were at M0.84?
In this case, ATC would expect you to forget about Mach no. and slow to 250 kts immediately. When ATC give an IAS 'on conversion', they assume that you will accelerate to that speed as you descend at fixed Mach and then maintain it.
If you had said something like, "we currently have 270kts indicated; would you like us to reduce to 250 now?" then ATC would probably have confirmed that. If 250 is too slow for you at that altitude, just tell them.
BTW, the phrase, "on transition" in this context was replaced some years ago with, "on conversion", to avoid confusion with transition altitude.
Last edited by eckhard; 8th Jun 2017 at 17:09.
Reason: Transition/conversion