To revise your example:
A: I have no idea how big my neighbor's house is, but I want you to paint it.
B: I measured it, and it is 1,200 square feet.  I can paint it for $2,500.
A: Agreed.
B: Whoops - turns out you owe me $5,000, because I asked the home owner how big it was, and they said it was only 1,200 square feet.  Turns out it's actually 2,400.
A: No.  Just, no.