The concept of master and slave are unrelated to the ability to send and receive data.
For example in a master/slave setup, the master can send data to a slave or it can ask a slave for some data. How this is done is down to the protocol that is used over the RS485 link. You could for example send a packet of data from the master to the slave where the 1st byte of the packet is a slave address, the 2nd byte is a packet identifier (or command), bytes 3 to n are the payload.
If you want to ask the slave for some data, then your packet would have a specific command in byte 2 so that maybe 0x01 = send me temperature, 0x02 = read ADC 1 value etc.