I think Armistead was freed in the 1780s a few years after the war, thanks to Lafayette’s efforts on his behalf, not after Washington’s death (Dec. 14, 1799).
Lafayette went to congress on Armistead’s behalf when Monroe was president. Monroe invited the Marquis de Lafayette to the USA and he ran into Armistead.
People ask why Armistead’s master didn’t just free him but its not quite so simple. The law that freed slaves who served in the war also forgave debts that the slaves were collateral for. Simply freeing a slave would be like selling the house that you had put up as collateral.