Return an array of results with a vlookup
 

I have a list of transactions with associated account numbers and transaction
dates; many of these account numbers appear multiple times.

Is there a way I can flag these line items according to the following logic?

If, within a given account, there are pairs of offsetting transactions (e.g.
+ $10 and - $10) that occur within 3 days of each other, flag those
particular transactions.

Thanks!
Singh