(CNN) Former astronaut Mark Kelly this week returned $55,000 he was paid for a speech in the United Arab Emirates last year at an event sponsored by the country's crown prince and attended by leaders of the country.

Kelly delivered the speech along with his twin brother and fellow astronaut Scott Kelly as part of a lecture series sponsored by the Crown Prince of Abu Dhabi, Sheikh Mohamed bin Zayed Al Nahyan.

Kelly is now a Democratic candidate in the 2020 US Senate race for the seat held by Republican Sen. Martha McSally of Arizona. Kelly has pledged to refuse corporate political action committee donations and to fight the influence of big money in politics as cornerstones of the early days of his campaign. The appearance could open Kelly to criticism of foreign influence now that he's running for the Senate.

CNN's KFile, which was reviewing Kelly's time on the paid speaking circuit, first reached out to Kelly's campaign last Thursday to inquire about his appearance in the UAE. An aide to Kelly's Senate campaign said he received a fee of approximately $55,000 from his speakers bureau, Keppler Speakers, who booked the appearance and paid Kelly through a consulting firm. The aide said Kelly decided to return the money this week to Keppler Speakers.

In early March, The Intercept reported Kelly made at least 19 paid appearances at corporate events across the US.