One way to do this is to in the vouchers table make a new field (credits_summary) that has the field type summary and is set to be a total of the credits field.
Then make a list type layout that shows records from the vouchers table. Delete the body part. Make a sub-summary part and select when sorted by voucher_number. In this part, place the fields voucher_number and credits_summary.
Show all records. Sort by voucher number.
The report should now display a list of voucher numbers and the total amount of credit for each voucher number.
For more on Walrus28's method see this tutorial on summary reports: Creating Filemaker Pro summary reports--Tutorial
It also possible to set up a table of vouchers, with one record per voucher number and use a relationship to link to the actual entries by voucher_number. Then, either the same summary field (in the transactions table, not the vouchers table) or aggregate functions such as Sum() can be used to get your totals by voucher.