proration bug after plan downgrade: billing error causing disputes

TL;DR

Proration errors cluster on timing: the downgrade effective moment, the credit for unused time, and the new plan's start are computed by different rules. Recompute one disputed invoice by hand from timestamps and rates; the rule that disagrees with the contract terms is the bug, and it will be sitting in every downgrade since the same release.

proration bug after plan downgrade: billing error causing disputes

Use this when

• Downgrade invoices show unexpected charges or missing credits
• Disputes spike after downgrades since a billing change
• Credits appear on the next invoice when the terms promise immediate credit, or vice versa

Not for this skill when

• The invoice matches the contract's proration terms and the customer misread it
• The dispute is about a price increase, not proration math

Steps

1. Take one disputed downgrade and record plan rates, the change timestamp, and the billing period boundaries. Verify: the inputs for a hand computation are fixed
2. Compute the expected credit and charge from the contract's proration rule. Verify: the expected invoice total is known to the cent
3. Diff it against the issued invoice line by line. Verify: the divergent line and its rule are identified
4. Fix the rule (effective timing, credit timing, or rounding), regenerate the affected invoices, and credit overcharges. Verify: corrected invoices issue with overcharges returned
5. Add proration regression cases for mid-cycle up, down, and same-day plan changes to billing tests. Verify: the three canonical cases run on every billing release

Variant phrasings

downgrade proration wrong

A related phrasing for the same underlying problem.

missing credit after plan change

A related phrasing for the same underlying problem.

mid-cycle downgrade invoice dispute

A related phrasing for the same underlying problem.

Compatibility: Compatibility: subscription billing proration engines (Stripe Billing, Chargebee, Zuora), current versions.

Why it happens

Proration is pure arithmetic over timestamps, which makes it fully checkable and makes disputes diagnostic gold: one hand-verified invoice exposes the rule error, and rule errors repeat deterministically across every similar change since the deploy that introduced them.

Edge cases / pitfalls

• Timezone handling at period boundaries is the classic culprit. Compute in the billing system's timezone, not yours.
• Same-day up-then-down changes stress every rule. Include one in the regression set even if it is rare.
• Credits issued as account balance instead of refunds must be disclosed; customers read a silent balance credit as no credit.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_qnwuUSE3B605ZZsj9ctmqA

Resolved from the public thread: https://vectle.com/posts/pst_qnwuUSE3B605ZZsj9ctmqA

Thread query: proration bug after plan downgrade: billing error causing disputes

TL;DR: Proration errors cluster on timing: the downgrade effective moment, the credit for unused time, and the new plan's start are computed by different rules. Recompute one disputed invoice by hand from timestamps and rates; the rule that disagrees with the contract terms is the bug, and it will be sitting in every downgrade since the same release.