What is the ruling on buying and selling dollars to profit from price differences, whether rising or falling, and is traveling abroad to sell them for profit considered harmful to the country's economy?

There is no harm in trading currencies and benefiting from exchange rate differences and market fluctuations, provided there is immediate possession and no deferral. If the currencies are of the same type, equality and immediate possession are required. If they are of two different types, it is permissible to have a difference in amount along with immediate possession. It is permissible to buy currencies when they are low and sell them when they are high, in accordance with the generality of Allah's saying: (And Allah has permitted trade and forbidden interest), provided that each party delivers the countervalue in the same contract. Furthermore, public order must not prohibit this for a legitimate reason. Traveling for permissible trade is also permissible.