United Press International's management and employes' union yesterday agreed to work together to find a buyer for the 78-year-old wire service, which has been operating under protection of the federal bankruptcy court since April.

UPI's hopes for reorganization lie in finding a buyer, but that goal has been endangered by continuing conflicts between labor and management.

The Wire Service Guild, UPI management and the company's investment adviser plan to form a committee to jointly select a buyer, the company and the union said last night after two days of talks.

Until now, the bankruptcy court has barred the union from presenting its own reorganization plan or proposing a new buyer, leaving those tasks to UPI's management and creditors.

Management asked the court last month to void the contract with the union after negotiations broke down between the two sides, which disagree over the need for further labor concessions in order to save UPI. Management later withdrew its request after a bankruptcy court judge appointed a federal mediator to supervise continued talks.

The union said Wednesday that it has hired the high-powered investment banker Brian M. Freeman to help it find a buyer for UPI. Freeman successfully represented Trans World Airlines' unions in their fight against Texas Air Corp.'s bid for TWA.

The Guild said it would ask a federal bankruptcy court judge to permit it to seek its own solution for the company's woes.

The decision to form a joint committee would head off the potential for conflicts between Freeman and the company's joint investment advisers, Bear, Stearns & Co. and Laddenburg Thalmann & Co.

UPI Chairman Luis Nogales yesterday praised the union's selection of Freeman as a "very positive" move that "will expedite the process" of finding a buyer.

He also said, when asked, that management has "no plans at this time" for resubmitting its petition to void the contract.