Make sure the post is 'square' to the floor. [=make sure that there is a right angle where the post and the floor meet]
I'll go on the trip if I can 'square it with' my parents. [=if my parents will agree to let me go]
Here's the rest of the money. Now we're 'square'. [=now we don't owe each other anything]
(..) Don't forget. Two shots apiece in the head as soon as you come out. How long do you think it will be before I can come back? At least a year. I'll square it with Mom, that you're not seeing her before you leave and... I'll get a message to that girlfriend when the time is right. Take care. (..)1972 The Godfather