How to load webpage content when HTTP POST triggers DOM changes?

Good afternoon,

I am writing a cross-site userscript for TamperMonkey. I am manipulating one page with jQuery and need to load information from another page using HTTP POST using GM_xmlhttpRequest. However, when accessing the requested page by browser, its DOM will be changed to its final state directly after the request. With the script's HTTP POST I only receive the HTML from before the change.

How could I get the page content from after the changes?