Does riba an-nasia (riba of delay) occur in this situation where the merchant counted out coins for the customer in preparation for exchanging them for another paper currency, and the customer left for a short distance then returned, whereupon the merchant completed the count and gave him what he was due in paper currency?

Counting money is not considered an offer and acceptance for a currency exchange contract, which requires immediate mutual possession (qabd). Qabd occurs after the completion of counting and agreement. As for things weighed and things counted, possession is not complete until their full quantity is received. The majority of jurists hold that immediate possession is not a condition for currency exchange; rather, what matters is not to disperse from the contract session, unlike the Malikis who stipulate immediacy. A man leaving for a short distance does not affect the ruling on the dispersal of the session. In summary, the questioner did not fall into riba al-nasī'ah (interest due to delay).