The bank doing the cash advance gives you the full amount (e.g. 50.000 pesos) . The cash advance is in pesos only. They will swipe your card (credit or debit) through a CardNet POS terminal. The receipt will say "CASH ADVANCE". Have your passport ready. They do not charge a fee on the transaction themselves, but your bank might, and they will split the fee.
Popular only does CREDIT CARDS
Progreso does CREDIT AND DEBIT cards
Banreservas does credit and debit but ONLY VISA cards
Cash advance is a good option if you need to withdraw larger amount of money. Many ATMs in the DR are now programmed to allow you to only take out RD$2.000 pesos per transaction when using card of a different bank than ATM owner.
Example: Popular, Progreso, Leon.
Also, many foreign cards will NOT work in some DR ATM's (don't know why...). I have both US debit Mastercard and European debit VISA, and neither works in Leon or Santa Cruz, but they work in Popular and Progreso.