Using a Script to pull value from sublist/search and insert in field

Hello all,

I am trying to develop a script that will get a field value to populate based on the total of values on a search/sublist.

Here is the scenario -

A custom field called "Committed Amount" exists on Custom Record A.
On a Custom Record B which is a child of Custom Record A, another custom field called "Amount" exists. Multiple Custom Records B will be created against Custom Record A. Intention is to total up the Amount fields for all child B records and insert that value in the field provided in A (Committed Amount).
A search is created (called "CR B Sublist") that has the "Sublist" view checked and "Show Totals" checked. The results show the total of the "Amount" field for all B records associated with A (for example 35,000).