He was approached to cash in his bet now that he had nearly reached the peak of the third mortal coil. This offer was to protect the person that ran the bet from making losses.
As it were, the person running the bet was unlikely to lose anymore because a lot of people had lost their bets. But he still wanted to earn more by making Arthur give up now.
If Arthur gave up on his bet, he would only earn half of what he should have won. He didn't want to, so he rejected the offer and decided to wait until he reached the fourth mortal coil to earn his full price.
He had bet 500,000 low-grade energy stones with odds of 20, so he is to earn 10,000,000 low-grade energy stones. If he accepted the offer now, he would only earn half of that.
Aside from this, the other significant thing that happened that concerned Arthur was that Thor also transcended the second mortal coil and entered the inner sect.
