The Hunt brothers of Texas have worked out final details of an $800 million line of credit to pay off their silver speculating debts and soon will begin using the money, sources familiar with the transaction said yesterday.

The loan, negotiated with the approval of Federal Reserve Board Chairman Paul A. Volcker, is expected to be a major topic at a round of hearings that opens today on Capitol Hill.

Carter administration officials responsible for regulating the silver market, representatives of commodity brokers who dealt with the Hunts, and Nelson Bunker and W. Herbert Hunt are among the witnesses called by a House subcommittee chaired by Rep. Ed Jones (D-Tenn.).

The Hunts originally were scheduled to testify on Thursday, but Jones yesterday agreed to let them postpone their appearance until next week because of scheduling conflicts.

The billionaire brothers are believed to have been the biggest losers when the price of silver collapsed two month ago. Both House and Senate committees are studying why the silver-speculating bubble burst and what ought to be done about it.

The $800 million is being loaned by a group of banks to Placid Oil Co., which is controlled by the Hunt family.

The loan will mean the Hunts won't have to sell their silver holdings to pay off their debts; if the Hunts wee forced to sell out, silver prices probably would fall even lower than they already have.