The local butcher, Sam Benton, was taking his savings but ...
but引导的是并列句,所以其前后两个分句都是独立的句子,都是能表达完整的意思的。但是,上面的这句话却没有表达出完整的意思,因为这句话只是表达了"Sam Benton正拿着自己的积蓄",但后面还缺状语才能表达完整意思。根据第45课的课文原文内容,我们可以确定,这个句子后面还缺一个地点状语to the post office。
三、正确答案
因此,上面划线部分的句子应该改为:
The local butcher, Sam Benton, was taking his savings to the post office but he lost his wallet.