Exchange-traded funds (ETFs) have been lauded for everything from democratising investment to boosting market liquidity. In Africa, savvy investors have hit on a less-obvious benefit: aiding their escape.

Foreigners are using a gold ETF to pull cash out of Nigeria amid the country’s ongoing dollar shortage. Portfolio managers are buying the Newgold Issuer ETF in the nation’s commercial capital, Lagos, using naira. Then they transfer holdings to the fund’s primary listing in SA and sell for rand...