Those banks received what many analysts saw as a break from Treasury in December, when it issued preliminary regulations governing the BEAT, which is part of the tax law’s overhaul of how the United States taxes companies that operate in more than one country. That overhaul has taken a political and public-opinion back seat to the corporate rate cut and changes in individual taxes, but it has drawn intense activity from business lobbyists seeking to shape the new rules to minimize their tax bills.

The BEAT is meant to curb a practice known as “earnings stripping,” in which multinationals avoid American taxes by shifting profits to other branches of the company operating in lower-tax countries overseas — often in the form of interest payments.

It is a sort of minimum tax, forcing companies that send their profits offshore to pay at least some American tax on them.

But in its December regulations, which provided nearly 46,000 words of details on how the provision applies to companies, Treasury essentially said certain interest payments made by foreign-owned banks are not subject to the calculations that determine that minimum tax. The move alarmed some former congressional aides who were involved in the tax effort. They said the exemption ran afoul of lawmakers’ intent in passing the tax overhaul.

The law’s authors tried to balance the international provisions to favor neither American-based companies nor foreign-owned ones. Throughout its drafting, they repeatedly asked the congressional Joint Tax Committee to run tax models to simulate the effects on both types of companies, eventually finding a near-50-50 balance. The Treasury regulations, which included the exemption that foreign banks had pushed for, could upset that balance.

Foreign banks received the regulations warmly but asked Treasury to go even further, in the name of fairness. The Institute of International Bankers, an industry lobbying group, told Treasury officials in a letter that it “wishes to express its appreciation for the strides made by the proposed regulations.”

The institute went on in the letter to push Treasury for further tweaks, in final regulations, that would reduce potential bank liability under the tax even more. Those highly technical changes would, if adopted, essentially exclude an even broader set of payments between banks and their foreign affiliates from the minimum tax calculations.