Dynamic currency conversion is a service offered by some merchants overseas, in which they convert your purchase to dollars so that you can better get a sense of what it costs. It might be easier for you to understand a price of, say, $35 compared with 500 South African rand. Using dynamic currency conversion will not affect your foreign transaction fee in most cases. (An exception is U.S. Bank, which reduces the fee to 2% when purchases are processed in U.S. dollars.)
In general, dynamic currency conversion is a costly convenience for U.S. travelers. A NerdWallet study of credit card currency conversion rates found that when you let your credit card company convert your purchase to dollars, it usually does so at the current market rate. But when you choose dynamic currency conversion, it typically costs you an extra 1% to 3% because the merchant uses a less favorable exchange rate and pockets the difference. In other words, rather than save you money, dynamic currency conversion could as much as double the surcharge on a foreign transaction.