He was named as the executor of his wife’s estate. The will called for the slaves to be manumitted when the estate’s debts were paid off.
So then he had legal control of them. Lee could sell them, rent them, or free them when he wanted, and not his wife. Sounds like he was sole owner to me.
And George Custis's will mandated the slaves be freed within five years after his death, not when the debts were paid off. Lee was a few months late in doing that, which is understandable since he was busy rebelling and all.