The best way to add a manual adjustment to costs is to add a manual bill and date it prior to your journal start date.
This is so that ScaleXP sees the cost but does not assume there is an associated bill already in your accounting system.
Go to Journals, Select Prepayments and click on the three dots (top right), then Add Manual Bill.

From here you can select an existing supplier or create a new one. You may wish to create a new "Adjustment" supplier for this purpose.
Create a bill for the adjustment amount, being sure to set the Bill Date to a date before you started using ScaleXP journals.
This is so that ScaleXP knows the bill does not already exist in your accounting system for the journaled month.
To add a bill to an existing supplier, you can also click on the existing supplier and then Add Manual Bill.

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