Why was Congress concerned about maintaining the balance between free
states and slave states?

Because without a proper balance between the two, the union may not have been established in first place,

Either because one of the two blocks - the slaves states and the free states- would have been set too powerful for the other to participate in the determination of public policy. Or because the blocks would have not agreed to join the union under such circumstances in first place.

no, the constitution said nothing about adding territory. president jefferson said he had authority to make treaties so he called the purchase a treaty.