Rachel S's answer is good, but not quite complete.
I believe the rest of the quote goes "if I could save the Union by freeing all the slaves I would do it, and if I could save the Union by freeing some, and leaving the rest alone, I would also do that".
Lincoln had campaigned on a platform that proposed preventing slavery's extension into the territories, rather than affecting the slaves already in captivity.
It was the belief, in many of the southern states, that Lincoln would move beyond that limited proposal toward a more radical proposal, such as might be promoted by abolitionists like Seward, his choice for Secretary of State.
Lincoln wanted to prevent the states from leaving the Union, if possible, and though he supported the abolitionist cause, he didn't know how to deprive slaveholders of what they believed was their property, except by freeing the slaves while simultaneously compensating the owners for their loss of "property". The estimate in 1860 was that there were some 4,344, 521 slaves in the southern states. At an average cost of $600 each, the cost to the taxpayers would have been $2,606,721,600. Excluding slaves, who were not taxed, the population of the United States in 1860 was 27,167,529, with a cost to every white man woman and child of about $95.95.
Such an option would have brought about a tax revolt north of the Ohio. The question was, how to free the slaves, without imposing a substantial burden on a significant portion of the voting public?
So, why would they be considering a financial burden to be more important than a moral one? It seems obvious to nearly all of us now, and to a substantial portion of the populace then, that the moral imperative was to free the slaves. But, most of the people who were of this opinion wouldn't be losing an asset appraised at approximately $600. It's quite likely that the $383.80 it would have cost a family of four to compensate the slaveholders, might make many moral men think twice about their convictions.
This is a very thin analogy, but when thinking of the cost of "going green" don't many people think we can get by a few more years without destroying the planet, at least until we can get the kids through college?