The $10,000 donation that cost Metro board member Christian Dorsey his position was returned to the agency’s largest union five months ago, but the check was never cashed — because it was lost in the mail, Dorsey and the union said.

Dorsey (D), an Arlington County Board member who served as one of Metro’s appointed leaders until February, lost that unpaid job over his failure to disclose the union donation within 10 days, as the Metro board’s ethics policy requires.

At the time of the donation, Dorsey was the chair of Metro’s Finance and Capital Committee. The money came from the Amalgamated Transit Union, which represents 13,000 workers and is a major player in the transit agency’s operations.

He promised in November to refund the money, and falsely said in January that he had done so. It was late February before he wrote the check.

The expenditure was not listed in Dorsey’s midyear campaign finance filing that was due July 15, however. Asked about it by The Washington Post, Dorsey said the exclusion was an oversight. He also said he had not noticed that the check, dated Feb. 21, was not cashed.

Dorsey later said he was told by the union that the check had been lost in the mail.

David Roscow, assistant to the president for communications at the union’s international headquarters, said that in late February or early March, numerous checks were mailed in a single envelope to the union’s bank.

But the envelope was damaged in the mail and never arrived at the bank. The Dorsey check was the only one of the bundle that was lost, Roscow said.

The five-figure refund came as Dorsey was struggling to recover from personal financial difficulties that included a bankruptcy in October.

After Dorsey solicited supporters for additional donations early this year, his campaign fund showed a balance of $11,203 as of June 30, following the deduction of the $10,000 refund.

“I have more than enough money in the campaign account to cover the return donation to ATU, and after fulfilling my commitment to return the contribution, we had no need to monitor when they deposited it,” Dorsey said in a text message, in response to a series of questions.

“To be clear, I did not ask, nor did ATU offer to delay processing the returned check. There was and is no need to do so,” Dorsey wrote. “We will gladly and immediately provide another reissue and have asked to directly transfer the funds to avoid any subsequent issues.”

Both Dorsey and the union provided images of the check, or its carbon copy, to The Post. Dorsey also provided a U.S. Postal Service receipt for a certified mail delivery Feb. 21.

The ATU money was among the largest campaign donations Dorsey has received during his public life, dwarfed only by money from the Democratic Party of Virginia and matched by the International Brotherhood of Electrical Workers.

Dorsey, 49, was first elected in 2015. He won reelection to the County Board in November, on the same day the Metro board met to discuss his failure to properly disclose the union donation. That matter, and Dorsey’s bankruptcy filing, were reported by The Post in the days after the election.

After losing his position on the Metro board Feb. 6, Dorsey also stepped down from the Northern Virginia Transportation Commission, which appoints Virginia’s Metro board member.

Dorsey said in November that he didn’t reveal the personal bankruptcy on the campaign trail because “the public’s interest is to see that I’m able to do the public’s business.”