The Dunmore’s Proclamation was a bill of attainder which stripped patriots of their property including slaves. The slaves themselves were given a narrow set of choices and as you say, the Brits reneged on their deal.
Given your speed and accuracy in responding to that post, I have a sneaking suspicion you were the Freeper who originally educated me about Dunmore’s Proclamation...
I am a history buff, but I concentrate more on some aspects/times of history than others, and this was a piece I had been oddly aware of until a Freeper brought it up in a thread.
It is those types of things that keep me a paying member of FR...:)