It's possible to use GetSummary in a script, but it's not possible to use it to find anything. Summary calculations are defined against the current found set; in Find mode there's no found set yet, so they don't evaluate to anything interesting.
I see two variants: a) establish a sorted found set and then loop through it, read subsummary for each group, and omit groups that are below the threshold, or b) do the math from the authors table. The latter looks somewhat simpler to me. I don't quite get the schema, but I suppose it's like that:
Author -< Title -< Payment
I.e. one author has many titles, and each title has many payments. We only need to consider outstanding payments, i.e. those that don't have “Yes” in their ‘To Pay’ field. Let's make a calculated field Outstanding Amount:
Case( To Pay ≠ "Yes", Amount )
Now we can summarize these fields from the Author table::
Sum( Payment::Outstanding Amount )
This calculated field is searchable. The idea of the script is to find all authors whose outstanding amount is greater than 100, Go to Related Records in the Payment table (matching all authors in the found set) and here are all the payments you're looking for.