As federal judges last spring began to consider the legality of President Trump’s punishing global tariffs, the administration offered an assurance: It told the court that a set of suing companies would “assuredly receive payment” if Mr. Trump ever lost the fight.

Now that the president has suffered such a stinging defeat, major businesses including Dyson, FedEx and L’Oreal have filed an early barrage of lawsuits in search of hefty tariff payouts. But the Trump administration has responded in recent days with a mix of dismissal and derision, setting up what may be a landmark legal battle over refunds.

At stake is more than $100 billion in revenue collected over the past year under a roster of tariffs that the Supreme Court invalidated last week. While a defiant Mr. Trump has since taken steps to try to revive the duties, he still faces the prospect that the money collected from his past tariffs might have to be paid back.

So far, roughly 900 claims seeking those refunds have been filed in federal court, according to the Liberty Justice Center, a legal group that represented some of the small businesses in the lawsuit that reached the nation’s justices. On Tuesday, the group took the first steps in two different courts to pave the way for affected firms to recover their past duties. The government must respond in one of those proceedings by Friday.

Despite its earlier assurances that it would repay the money, the Trump administration has now signaled it may be ready for a fight. In the wake of the Supreme Court’s ruling, Mr. Trump brushed off any refunds as unsettled legal ground, as he wagered aloud that a resolution could take “years.” Scott Bessent, the Treasury secretary, said on Tuesday that refund requests could not even start for about a month.

“We will see what the lower court says, and we will follow what the lower court says to do,” Mr. Bessent said in an interview on NBC.