in British pay 中的 pay 为何用原型

A German army in British pay, and commanded by the “Butcher” hero of Culloden, was beaten in July, and capitulated in September.

请问句中的 pay 为何用原型？

本句来自1857年12月12日美国Daily Union报中的一篇题目为British India的文章。

A German army in British pay, and commanded by the “Butcher” hero of Culloden, was beaten in July, and capitulated in September.

这里pay为名词，而不是动词。

in the pay of somebody/something为习语，意为为某人或某机构工作（一般为秘密工作）。一般带有贬义。

working for somebody or for an organization, often secretly.

例句：

He was in the pay of the drugs barons. 他为一些大毒枭工作。

in British pay=in the pay of Britain.

A German army in British pay: 一只替英国打仗的德国军队。