What is the currency exchange policy in term of currency denomination of US Dollar?

US dollar of denomination of 100 bill/note fetches slightly higher rate than 50 dollar and below denomination. For instance, as of today, 28.10.2013, the prevailing rate for 100 bill/note is Ngultrum (The currency of Bhutan) 82.93 whereas the rate for the 50 dollar and below.Overall, the rate of US dollar is subjected to change at time with Ngultrum, which is pegged at par with Indian Rupee