If Rainsford manages to win the hunt, Zaroff promises Rainsford freedom.
I would like to clarify the above statement a bit. Rainsford doesn't actually have to beat Zaroff by killing him. Rainsford wins if he is able to avoid being killed by Zaroff for three days. Rainsford is the prey, and Zaroff is the predator. Zaroff's agreement to every prey of his is that if the prey survives for 72 hours, then the prey wins the game.
"If my quarry eludes me for three whole days, he wins the game."
When it is Rainsford's turn to be hunted, Rainsford asks for some further clarification about what surviving and winning will actually get him. Zaroff gives his word as "a gentleman and a sportsman" that Rainsford will get to leave the island alive on Zaroff's boat.
"And if I win--" began Rainsford huskily.
"I'll cheerfully acknowledge myself defeat if I do not find you by midnight of the third day," said General Zaroff. "My sloop will place you on the mainland near a town."
Readers never get to see if Zaroff would actually keep his word, because Rainsford kills Zaroff. Rainsford doesn't give Zaroff the chance to keep the promise.