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

I am trying to capture the date that an agent reaches their commission cap so that commission formulas change after that date. I have two tables: Agents and Transactions - Agents is the parent table and Transactions is the child table. I created a summary field in Agents that totals the agent's commission across the related Transactions records [Total Commission to Broker]. When the summary field hits a certain number then I want the date that it hit the number captured so that my formulas in the Transactions table will adjust to stop collecting commission for the broker after that date. I can't use a form rule because the summary field is in the Parent table but the person entering data will be doing so in the Transaction record. I also tried automations but it won't let me save the automation when I select the same table or a parent table. Any help would be really appreciated!