Can you try this?
SUM(if year(tran date)=2016 then [Percentage of Budget used] end)*SUM(if year(tran date)=2017 then [Budget Amount] end)
How about something like:
SUM([Budget Amount]) * (LOOKUP(SUM([Percentage of Budget Used]),-1))
Just make sure you're computing table across, and correct any aggregations as needed (e.g. the fields may already be aggregated or may use an aggregation other than SUM).