"I was hoping she wasn't joking," he told CBS News correspondent Randall Pinkston.
Rivera and seventh-grader Brianna Tan are part of an experiment that pays some students for academic achievement.
Brianna told Pinkston she has earned close to $50.
It's called the "Spark Program." In New York, 8,000 randomly selected fourth- and seventh-graders earn for learning.
Fourth graders can earn up to $25 for scoring well on 10 English and math tests for a maximum of $250; seventh graders can make $50 per test - or up to $500 a year.
Similar programs are underway in Chicago and Washington, D.C.
"It is getting kids on the right path to do the things that we want them to do," said Harvard economist Roland Fryer.
The controversial privately funded program, the brainchild of Fryer, is drawing high-profile attention - even from Stephen Colbert.
Formal analysis is due this fall, but the anecdotal evidence is promising.
Last year, just 60 percent of Roy Rivera's third-grade class met state math standards. This year, with cash rewards, 84 percent made the grade.
Despite positive preliminary results, critics say the very idea of paying students to study is risky business.
"Once you create this expectation in students, how do you unwind it?" said Heather MacDonald, a fellow at the Manhattan Institute. "When do we stop paying them? We're just bribing students to do something they should do on their own."
But advocates say the two-year pilot program is intended to teach kids early that hard work pays off.
"During that year, kids develop habits. They see what they can do when they apply themselves with more energy and focus, and that carries over," said Mary Pree, assistant principal at P.S. 188 in New York.
They are habits that educators hope will last a lifetime.