A provision in the new tax law that takes effect Thursday has touched off an unprecedented bidding war by American corporations to acquire tax benefits from money-losing businesses that have more such benefits than they can use.

The leasing provision in President Reagan's tax law permits profitable companies to "buy" tax credits and deductions from unprofitable firms by purchasing some of the firms' equipment or other assets and leasing them back to the sellers.

The price will be "cash on the barrelhead," said Richard G. Bentley, assistant controller of Ford Motor Co., one of the unprofitable companies that hopes to make full use of the new provision.

Tax experts and lawyers are being besieged by corporate executives seeking to make a deal. One Wall Street firm has canceled vacations to send its executives around the country in a matchmaking blitz.

In effect, officials at such currently unprofitable firms as Ford, United Airlines' parent, UAL Inc., International Harvester and Chrysler Corp. are being courted by the hour, as profitable companies seek indications of how much they will have to bid to buy the other company's tax benefits.

Ford's Bentley said yesterday that he is talking to a number of companies and that "some offers and counteroffers are being traded back and forth."

What Ford has to offer is some $70 million of investments made this year, for which it cannot claim tax writeoffs because it is headed for another large loss.

Financial community sources say the initial payment by Ford's profitable buyer is expected to range from 14 to 25 percent, and Bentley said he obviously prefers the higher figure.

If Ford were to get 20 percent on $70 million of investments, for example, it would mean an overnight transfusion of $14 million.

Chrysler has about $100 million of investment credits for sale, from which it could get $20 million or more.

Administration officials hope much of the money going to weaker firms will be used for plant modernization. In addition, it is possible that a bankrupt firm such as Washington's Auto-Train Corp. could get its passenger trains running again between Northern Virginia and Florida by working out a deal to sell its benefits in return for some immediate cash.

"It's the hottest area of business today," said Jeffrey Puma, a tax specialist in Washington with the big accounting firm of Peat, Marwick, Mitchell & Co.

The pace of negotiations has become furious in recent days because the new tax law provides that all credits for investment spending in 1981 are up for grabs retroactive to Jan. 1, but only if agreements are signed by Nov. 12.

At stake are untold billions of dollars in benefits to profitable and money-losing businesses alike, and in fees to accountants, lawyers and others that have set up shop as middlemen for the new bidding. General Electric Credit Corp., the financing subsidiary of GE, is one of the middlemen.

Officially, the administration has estimated that tax revenues lost because of the new provisions will total $27 billion over the next six years. However, one congressional estimate puts the loss at some $31 billion, and tax specialists said the flurry of negotiations in recent days may indicate a substantial increase in the amount of lost revenues. This could complicate the administration's short-term and long-range budget planning.

Alan Greenspan, who headed the Council of Economic Advisers under President Ford, says the new tax provisions are "sort of the equivalent of food stamps for undernourished corporations," allowing subsidies for sectors of the economy that the private markets wouldn't support.

And one prominent financier, who said he is aware of many negotiations "on a very big scale," expressed concern about a "stormy reaction" to diminishing levels of corporate taxation in the next few years because of the leasing provision and other sections of the Economic Recovery Tax Act.

But Ernest Christian, a strong supporter of the new rules and a tax lawyer with the firm of Patton, Boggs & Blow, predicted yesterday that the leasing provisions "will prove in time to be the crucial element in causing the increased capital investment" envisioned in the tax law overall.

Since the Treasury Department is not expected to issue a draft of proposed regulations covering the benefits until early October, tax specialists and their client corporations will have less than six weeks to reach financial agreements before the Nov. 12 deadline.

After that, the new law will apply only to subsequent business investments, not to earlier 1981 spending.

According to Puma, of Peat Marwick, the transaction does not require much more than a paper exchange. An unprofitable firm that owns assets purchased in 1981, usually acquired through outside financing with a manufacturer or creditor, sells those assets to a profitable firm in a deal called a "sale-leaseback."

Part of the cost of the assets is paid initially in cash, and the balance becomes a note that requires no actual payments since principal and interest payments to the profitable firm exactly match the purchase installments due the original seller.

Thus, if unprofitable company A sells $100 million of assets to company B, and company B agrees to pay 20 percent in cash immediately, the troubled firm gets $20 million on the spot and company B gets current and future tax benefits from owning the $100 million of goods: a 10 percent investment tax credit worth $10 million in reduced taxes plus a 15 percent depreciation writeoff that translates into about $7 million in tax reductions, for a total tax reduction of $17 million over the first year.