这个 it 是指of you to have given us so much money，而不仅仅是to have given us so much money。其中的 you 为不定式的逻辑主语，说话者指的是“你给了我们那么多钱”。

类似地，下面句各句中的 it 也是形式主语，也是用于指代 of sb to do sth 或 of sb to have done sth：

It was funny of Sam to do that. 萨姆干那事真怪。

It was kind of you to help him. 你帮助了他，真好心。

It was kind of him to wait. 多蒙他好心等候。

It was kind of him to speak to us. 他跟我们说话，真是好心。

It was generous of him to pay us. 他付了我们钱，真慷慨。

It was good of you to phone. 你那个电话打得好。

It’s very good of you to have listened to me. 感谢你能听我讲。