What trick does Sergeant Troy play on Mr. Boldwood in Chapter 34?

When Troy reenters town, Boldwood corners him and addresses the matter of Fanny Robin. Boldwood offers Troy over 500 pounds if he will make an honest woman of Fanny by marrying her. When Boldwood discovers Troy has slept with Bathsheba, he offers Troy the same amount to marry her. Then Troy declares they’re already wed.