Did she mention he gave them their freedom and they never left the plantation.
Not exactly.
In his will Washington gave his slaves their freedom on the death of his wife.
Martha shortly after Washington’s death freed those slaves after reading of slaves murdering their masters in the Carolina.
She did not want the slaves to have a compelling reason to murder her.
Yes many of the slave remained on Washington’s plantation as wage workers. Being in Virginia it would be a long walk to a state where slavery was illegal.