He is motivated to continue his fruitless search by the promise of a large reward, recently doubled, upon the letter's safe return, and he will pay 50,000 francs to anyone who can help him.
他曾被承诺在信件被安全归还后，会得到一大笔奖金，而这笔奖金最近又被加倍了，并且他愿意付50000法郎给任何能帮助他的人。