Final answer:
Ana should adjust the Field-Level Security for the Account.Purchase_Amount__c field in the Salesforce Setup by configuring the correct access for the relevant profiles or permission sets. This is done directly in the Salesforce interface, and a User Story is not necessary unless the changes need to be tracked or are part of a larger deployment process.
Step-by-step explanation:
To grant Field-Level Security (FLS) to the Account.Purchase_Amount__c field without affecting any work in progress, Ana should follow these steps:
Since Ana is not interested in committing any work in progress if she is using a version control system or a deployment tool, she should simply make the necessary FLS changes directly in Salesforce setup without including it in any User Story.