If I pay for a Laos visa on arrival at a land border with a US $100 note will they give change in USD?

Yes, at least they did for me 10 days ago.

You'll need clean, unmarked, pristine bills, no wrinkles. Indeed, I have a friend heading back to the U.S. next month, sending some used bills with him to exchange and bring back usable ones.

Mac

Thanks Mac

You would be better off with a nice clean unmarked $20 or two new $10 bills in Vientianne.

Last time I crossed into Laos, I paid 1500 Baht and they gave me 250 Baht in change, so there was no need to take US$

Yes, they will give change. However, it's better if you can provide the exact change, or failing that, at least give them something smaller, like a US$50 note.

Depends on the border crossing too - if you're heading across the 1st Friendship Bridge to Vientiane, there won't be any issue as they always have lots of people crossing and there's even a couple of moneychangers and ATMs. If you happened to be crossing at a more isolated crossing like Huay Kon in Nan or Thai Li in Loei, then you'd be better off with the exact change in case they don't have any.