Million and billion in suffix of map legend

Dear Members,
I am plotting the population using style= "fixed" with breaks, I want my
legend to show "mn"  and "bn"  as a suffix.
 tm_fill(legend.format=list(fun=function(x) paste0(formatC(x,
digits=000000, format="big.num.abbr"), "mn")))
I tried with the above codes with tmap but could not succeed.
Please help.
Thanking you in advance.

Raman Mishra
Senior Research Scholar
International Institute for Population Sciences, Mumbai
Mobile: +91-8879317106