China’s foreign ministry has spent weeks denying that detained Canadians are, in effect, being held for ransom against Ms Meng’s release. 

【翻译】中国外交部几周以来一直否认，被拘捕的加拿大人实际是为了要求释放孟女士而被拘留换取赎金的。

against 有一个特殊用法，就是“在......背景下；针对......而来的”：

If something happens or is considered against a particular background of events, it is considered in relation to those events, because those events are relevant to it. 在 (某事件背景) 下。