SP Online Lookup SUM of Another List

jonboylib This is not possible using out of the box capabilities of SharePoint.

You need customized solution for this, either using Power apps or power automate. You can run a power automate flow on item creation/update in Purchases list which will get the list items using "Get list items" action (you can apply Status equal to Complete & code equal to 123 filter in action).

Then you can get the sum of amount using expressions in power automate flow using which you can add/update the item in Budget list (using Create item or Update item actions in flow).

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