AMSTERDAM, Netherlands — African e-commerce company Jumia is trying to encourage more prepayment to discourage returns or cancellations, the head of its Nigerian business said after a report last week by Citron Research questioned some of Jumia's sales figures.

Jumia became the first African tech stock to list on Wall Street on April 12. Its shares initially soared, but fell sharply on Friday after the publication of the report by Citron Research, run by short-seller Andrew Left.

"We stand by what we disclosed... the way GMV (gross merchandise value) is calculated in the industry is gross of cancellations and returns," Juliet Anammah, chief executive of Jumia Nigeria, told Reuters by telephone after cancelling a trip to address a retail event in Amsterdam at short notice.

She said many customers in Nigeria, Jumia's biggest market, still only pay by cash when they receive their orders, but Jumia was trying to move customers to its Jumia Pay solution to pay in advance when they check out online.

Loss-making Jumia was making progress toward reaching its goal of breakeven by the end of 2022, Anammah added.

It plans to make its marketing spending more efficient, charge merchants for storing their goods in its warehouses, boost sales of advertising on is site and charge sellers to create content, such as images of their products, she said.

"We are going to monetise value-added services such as Jumia Express and add on more advertising. We have just started a whole lot of opportunities," she said.

Jumia shares were trading 3.5 percent higher at $27.52 by 16:58 GMT after two brokerages raised their ratings.

By Emma Thomasson; editors: Georgina Prodhan and Alexander Smith.