RS485 communication between Arduino
From the description that you gave in the question it seems like at the Mega 2560 end did not put its end of the link into receive mode after sending the request. 

RS485 is a Half Duplex protocol and you can only send one direction at a time. If the MEGA 2560 is still in transmit mode after sending the request it would be causing a collision on the bus wires when the NANO is trying to send the value back. If you looked at the bus lines with an oscilloscope you could confirm the collision.