In the below substitute HL4140CN with the printer name. (also the option names may difer, this is for a brother)

List the options with:

lpoptions -p HL4140CN -l

Add an “instance” of the printer with the option for Mono printing

lpoptions -p HL4140CN/bw -o BRMonoColor=Mono

Now the following will print Mono

lpr -P HL4140CN/bw any.txt