I googled it and finally I found the answer to my problem. All you need to do is to reset the purge counter. These steps worked in MFC620CN/420CN/410CN/210C/240C and FAX2440C model.

To reset purge counter

1) Enter machine maintenance mode (on MFC240C all four photo/copy/fax/scan lights flash in maintenance mode). Care is needed in maintenance mode, otherwise critical settings may be inadvertently corrupted.

FAX models with numerical keypads: Press the Menu/Set, *, 2, 8, 6 and 4 keys within two seconds. (if you fail to complete the correct key sequence within two seconds, press the Stop key to try again).

Other (non-FAX) models: Press the Menu/Set and Black Start keys. Next press the “up arrow” key four times to enter the maintenance mode).

2) Press the 8 then 0 keys in the initial stage of the maintenance mode to call up the machine’s log information list (function code 80).

3) Press the Black Start key several times to scroll through the log information list to reach the purge counter. The purge counter is the 25th item in the list.

4) Press the 2, 7, 8, and 3 keys in this order to reset the purge count to zero.

5) Press the Stop/Exit key to return to the initial stage of the maintenance mode.

6) Press the 9 key twice to return to the standby state.

Source: Fixyourownprinter.com