What did the United States government gain in return for allowing American sugar planters in Hawaii to sell their sugar in the United States without paying import duties in 1887?

The right to use Pearl Harbor as a naval base.

Explanation:

For several decades, the sugar farmers in Hawaii had been economically disadvantaged by United States import taxes tariffs placed on their farm product, and as a result had been making efforts to negotiate for a free trade agreement. There had been two previous attempts at reaching an agreement with the United States which all failed, due to many reasons. The planters sought after a treaty, but the Hawaiian citizens were afraid it would pilot to annexation by the United States.

In the year 1872 The Reciprocity Treaty was signed which allowed Hawaii to trade sugar with the United States without any paying of taxes, duties or tariffs, this greatly increasing plantation profits for the Hawaiians and also gave the United States the right to use Pearl Harbor as a naval base.

it granted civil rights to african americans.