A bankruptcy trustee is suing former Trump campaign manager Paul Manafort over a failed real-estate deal involving the political consultant’s former son-in-law.

The trustee, Thomas Casey, sued Mr. Manafort on Thursday in an Orange County, Calif., bankruptcy court over Mr. Manafort’s $2.7 million investment in a renovated Los Angeles mansion, asking a federal judge to determine that the payment was a capital contribution, not a loan.

Mr. Casey said Mr. Manafort’s investment was used to help buy the Bel-Air property for $8.5 million in 2016, one of several failed real-estate projects led by Jeffrey Yohai, Mr. Manafort’s former son-in-law.

Mr. Casey was appointed in December to sell the property and distribute the proceeds to pay off the project’s final bills. When small businesses collapse into bankruptcy, the money collected from property sales is distributed to secured lenders before lower-ranking investors.

Recasting Mr. Manafort’s $2.7 million investment as a capital contribution could mean the payment would be considered as equity, and thus only paid after other creditors are paid in full.