Correct me if I'm wrong, but I'm pretty sure the amount of money you get when selling Legion and NCR money is dependent on your Barter skill. If this is true, the easiest way to fairly exchange your currency without 100 Barter is this:

First, go to any casino. Primm is the first one you have access to, obviously, but it isn't open yet. In order to reopen it, first complete My Kind of Town. Then leave and wait 3 days. When you go back in, an NCR deserter and her 3 minions will confront you in the casino. There are several ways to deal with them. After that, leave and wait another 3 days and the casino will open.

The rest of the casinos are open by default.

After you do that, go to the cashiers that let you exchange money for chips, and exchange all of your non-cap currency (NCR and Legion) for chips. After you've done that, just cash in all your chips.

Now, I'm not sure if NCR and Legion money values are locked when buying/selling them. They may be. But if they are, the casinos still have infinite money, so you don't have to go from vendor to vendor if you have a lot of NCR or Legion money.

Hope this helped!