Dynamically changing the href on a link with data-dialog-type = modal does not work
Presumably, the JS used to generate the dynamic href is on the page that the modal opens?  If I'm not mistaken, iframes do not run their attached scripts by default.  So this page may be helpful:

https://stackoverflow.com/questions/251420/invoking-javascript-code-in-an-iframe-from-the-parent-page?utm_medium=organic&amp;utm_source=google_rich_qa&amp;utm_campaign=google_rich_qa

Since your requested iframe is on the same domain as your parent page, there shouldn't be any CORS issue.