If you're planning to bring U.S. dollars to Paraguay and exchange them for Paraguayan Guaraníes (PYG), there are some important restrictions and best practices to keep in mind to avoid problems at exchange offices or banks.
Paraguayan exchange offices strongly prefer new series U.S. banknotes, especially in $100 and $50 denominations. These are easier to verify and are widely accepted without issue.

Old-design bills—even if technically still legal tender in the U.S.—are often rejected or devalued in Paraguay. In addition, bills that are torn, heavily creased, stained, or have any writing or stamps may also be refused or exchanged at a lower rate.

Bringing the right kind of cash ensures a smooth exchange process and avoids unpleasant surprises. If in doubt, check your bills before traveling or ask your bank for newly issued notes.