The Admin wrote the formula backwards relative to how Validation Rules actually evaluate. A Validation Rule's error condition formula must evaluate to TRUE specifically for the BAD data that should be rejected, not for the good data that should be allowed through. The formula Amount > 0 evaluates to TRUE whenever Amount is a valid positive number, meaning the rule fires its error precisely on every legitimate, correctly-entered record, while a record with Amount at zero or negative, which is actually the invalid case, would evaluate to FALSE and save without triggering any error at all — the exact opposite of the intended behavior. The correct formula should describe the invalid condition directly, such as Amount <= 0, which only evaluates TRUE when the data is genuinely bad, correctly blocking only the records that should be blocked.

Without ISCHANGED(StageName), a Validation Rule checking the current Stage value would re-evaluate and potentially fire its error on EVERY save of that record, even when a user is editing a completely unrelated field like Description or a custom text field, and Stage itself was never touched during that particular save. This creates a frustrating experience where users are blocked by a Stage-related validation error while trying to make an entirely unrelated edit, since the rule cannot distinguish between "Stage is currently at this value" and "Stage was JUST changed TO this value during this specific save." Wrapping the relevant Stage condition in ISCHANGED(StageName) ensures the rule's Stage-specific logic only evaluates when Stage itself was actually part of the current save's changes, correctly scoping the validation to genuine Stage transitions rather than firing indiscriminately on any edit to the record.

The safest approach is a Custom Permission-based bypass rather than temporarily deactivating the Validation Rule entirely. This involves creating a Custom Permission, assigning it via a dedicated Permission Set to only the specific integration user or migration process performing this load, and modifying the Validation Rule's formula to include a check like AND(NOT($Permission.Bypass_Validation_Rules), [original condition]), so the rule only skips its check for users who explicitly have that Custom Permission assigned. This is significantly safer than simply unchecking the rule's Active status during the load window, because a full deactivation removes the rule's protection for EVERY user in the org during that period, including regular users entering data through the normal UI, potentially allowing bad data in through the front door precisely while the safeguard is disabled. The Custom Permission approach scopes the exception narrowly to only the migration process that genuinely needs it, leaving normal data entry fully protected throughout.

No, this cannot be implemented as a Validation Rule, because a Validation Rule has exactly one capability: evaluating a condition and either allowing the save to proceed or blocking it with an error message — it has no ability to WRITE or modify any field's value, including the very record it is evaluating. Automatically SETTING Probability to a specific value based on Stage is fundamentally a write operation, which is structurally outside what any Validation Rule, or any pure formula-based mechanism, can perform. The correct tool for this requirement is a before-save Record-Triggered Flow, covered starting in Module 13 and 15, which specifically can detect that Stage has changed to Closed Won and then update the Probability field on that same record as part of the save process, something no Validation Rule can ever do since it lacks any write capability entirely.

This happens because of the Order of Execution timing between before-save Flows and Validation Rules: before-save Record-Triggered Flows run BEFORE Validation Rules evaluate, meaning by the time the Validation Rule checks whether Discount is blank, the before-save Flow has already run and set it to 0, so the field is no longer genuinely blank from the Validation Rule's perspective. The Validation Rule is not malfunctioning — it is correctly evaluating the record's actual state at the point it runs, which reflects the ALREADY-DEFAULTED value from the Flow, not the raw, originally blank value the user actually submitted. If the business genuinely wants to reject blank submissions rather than silently defaulting them, either the before-save Flow's defaulting logic needs to be reconsidered, or the Validation Rule needs to check a condition the Flow does not overwrite, since checking for blank Discount after a Flow already fills it in will structurally never catch the original blank submission.

This requires several elements combined with AND logic across the whole rule. First, a bypass check using NOT($Permission.Bypass_Validation_Rules) must wrap the entire condition, since if this Custom Permission is assigned, the rule should never fire regardless of any other condition, directly addressing the migration/integration exception scenario. Second, ISCHANGED(StageName) scopes the rule so it only evaluates when Stage itself was actually part of the current save, preventing the rule from firing on unrelated edits to records already sitting at Negotiation or later. Third, an OR combining StageName equals "Negotiation" and StageName equals "Closed Won" (or additional later stages as needed) captures the "Negotiation or later" requirement, since a simple equals check against one Stage alone would not cover multiple qualifying stages. Fourth, an OR combining ISBLANK(Amount) with Amount less than or equal to zero correctly catches BOTH ways Amount could be invalid — genuinely blank, or present but zero/negative — mirroring the blank-field guarding principle from Formula Fields in Module 11, since checking Amount <= 0 alone would not catch a truly blank field.