When running the report to show a Donor's contribution for a specific time period,the amount is overstated because it is not showing transactions that will reduce it. The transactions are generally either Credit Memos or General Journal Entries and were used for returned NSF checks.
By default the Donor Contribution Summary report only shows two types of transactions, Payment and Sales Receipt. For it to show other transactions we have to modify the report. To do this follow these instructions: