The Puerto Rican government, scrambling to avert the U.S. territory's impending default, is facing unexpected opposition from the Wal-Mart chain, which is suing the commonwealth in an attempt to overturn new taxes the retailer claims are discriminatory and violate the Constitution and federal law.

With its lawsuit, the Puerto Rican arm of the company is going after Act 72, which Gov. Alejandro García Padilla signed in May. The bill increased the "tangible property component" of the corporate alternative minimum tax, Forbes reported. In practical terms, the local alternative minimum tax applies whenever the "tentative minimum tax" exceeds the "regular" income tax, meaning that taxpayers are always stuck with the higher amount.

Act 72 also increased to 6.5 percent the tax on goods imported from offshore affiliates to local companies with gross revenues of more than $2.75 billion, Bloomberg noted. The tax had previously been at merely 2 percent, the business publication added.

"No government should be permitted to drive a company - the largest private employer - out of business through a special tax applicable at its highest rate only to that company," Wal-Mart said in its Friday filing, according to CNN Money, adding that it estimated that the tax would force it to hand over 91.5 percent of its net income to the commonwealth.

That rate is "three times the average effective tax rate that Walmart's affiliated companies pay worldwide," the company added in the statement.

The government in San Juan, meanwhile, continues in its efforts to try to restructure $70 billion in debt, an amount that exceeds that of every U.S. state except New York and California, Bloomberg noted. Guggenheim Partners, an investment and advisory financial services firm, warned its clients that the commonwealth's attempt to get help from the federal government may not go too far, CNN Monday added.

"We continue to question if Washington is prepared to offer material help to Puerto Rico. This suggests more defaults are likely as the Puerto Rican government appears to lack the ability to meet its debt obligations," Guggenheim's Jaret Seiberg detailed in a note.