he had to pay other five pounds out of his pocket为什么要把other换 another

因为 "another + 数词'= "数词 + more",表示“再……,又……”,如：
another two cakes = two more cakes 再两个蛋糕
another four books = four more books 再四本书
这里的another不能改成other,退一步讲,other跟数词搭配的顺序也应该是“数词+other”,而不能“other+数词”,如：five other people其他5个人