How to capture the date (or date/time) a summary field reaches a certain number

I have an alternate suggestion.  Lookup that summary total down to the detail records and call it like [Total sales (lookup)], and then make a new field to be a snapshot called [Total sales at time of sale].

Then your formula can simply key off of what the total sales were at the time of the sale (counting that sale).

Here is some help in setting up snapshot fields.

https://help.quickbase.com/user-assistance/setting_up_snapshot_fields.html