You can use a tag manager to dynamically inject Affirm-related scripts onto your page.
Google Tag Manager
GTM includes a feature to sequence tag execution, so that you can make sure that Affirm JS is always loaded first.
Here's an example of how that would be setup:
|Tag 1||Affirm JS embed code||All pages||Page load (auto)|
|Tag 2||Affirm promo script||Only pages where promo should be displayed||Fires after 'Tag 1'|
And here is a screenshot of how these settings look in GTM: