This behavior occurs when the Opportunity Close Date falls within the “Next Donation Date Match Range” defined in NPSP settings.
When this match happens:
The Opportunity Amount is automatically updated to match the Recurring Donation installment amount.
The Opportunity Close Date may also be updated to the installment’s Next Donation Date.
No new Opportunity installment is created for that period; instead, the existing Opportunity is reused and synced.
This behavior is expected and controlled by the Next Donation Date Match Range setting in NPSP.
To prevent unintended Opportunity Amount updates:
Review and adjust the Next Donation Date Match Range value in NPSP Settings.
Ensure manually created Opportunities linked to Recurring Donations have Close Dates outside the configured match range if they should remain independent.
Be aware that Opportunities within the match range will be reused and synced rather than creating a new installment Opportunity.
https://help.salesforce.com/s/articleView?id=sfdo.npsp_configure_recurring_donations.htm&type=5
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