Explanation
To require the Amount field during the negotiation stage for sales users but allow their managers to make changes, the administrator should configure a validation rule that checks if the user profile is not a sales manager, the stage is negotiation, and the amount is blank. This will prevent sales users from saving the record without entering an amount, but allow sales managers to do so. Making the field required for all users will not meet the requirement. Creating a formula field or assigning the administrator profile to the managers will not affect field requirement. References:
https://help.salesforce.com/s/articleView?id=sf.validation_rules_overview.htm&type=5