You don't seem aware that Parke Custis left cash "legacies" for some of his female heirs for which the cash did not exist. Lee was stuck trying to create the cash to satisfy his father-in-law's will. That is why he tried to extend the emancipation order in the will.
"And upon the legacies to my four granddaughters being paid, and my estates that are required to pay the said legacies, being clear of debts, then I give freedom to my slaves, the said slaves to be emancipated by my executors in such manner as to my executors may seem most expedient and proper, the said emancipation to be accomplished in not exceeding five years from the time of my decease."
The will stipulated that he should sell his estates (his land) to pay the bequests, and free his slaves as quickly as he could. Lee did not sell any of Custis's estates to pay the legacies, nor did he give freedom to his slaves in an expedient or proper fashion. With his pro-slavery stance, it would be easier to try and fight to keep those slaves as long as he could to rent out or work the land to make money and if it took longer, so be it.