If this is Adagio Receivables, it would be far easier to issue a Credit Note than an Adjustment entry. From the Enter Invoices function, create a new entry, and change the 'document type' from Invoice to Credit Note. You can then choose the particular invoice you wish to credit. A $200 credit note will reduce the invoice amount by $200.

Adjustment entry can sometimes be deceiving. In this case, you needed to enter an amount in the 'debit' field to reduce the amount owing on a Receivables invoice.