Originally Posted by
B747-437B
No problem at all. Just keep in mind that immigration only accepts US$50 and US$100 bills of 2008 or newer.
Should follow up that in DAR this bolded part is not true in my experience - I've regularly paid with $10s and $20s.
However they are sticklers for new-looking bills with no tears or other blemishes whatsoever. (They will often try to return as change such bills, though, which should not be accepted if you want to use it on another entry... "Can you please give me a newer bill" works fine in that case.)
If you don't have USD in DAR, they'll invite you to head outside to the ATMs and money changers. Can't speak for other airports.