Question 58582
IF ADAM OWNS THE $340000 PROPERTY & GARY OWNS THE $ 420000 PROPERTY AND THEY SWAP. THEN ADAM OWES GARY THE DIFFERENCE IN VALUE OR $420000-$340000=$80000. 
PROOF
GARY STARTED WITH $420000 WORTH OF PROPERTY
ADAM STARTED WITH $340000 WORTH OF PROPERTY
GARY ENDED UP WITH PROPERTY WORTH $ 340000 AND RECEIVED $80000 FROM ADAM. SO HIS NET WORTH IS NOW $340000+$80000=$420000.
ADAM ENDED UP WITH PROPERTY WORTH $420000 AND PAID $80000 TO GARY. SO HIS NET WORTH IS NOW $420000-$8000=$320000.