One possible cause of this issue is the duplication of number sequences. In Project management and accounting, there are separate number sequence setups available for On-Account Invoice, Invoice, On-Account Credit Note and Credit Note document types. If the number sequences are not uniquely defined, in the posting process, when the records are fetched from the projProposalJour\ProjInvoiceJour it is possible that posting a credit would fetch the corresponding invoice with the same number sequence and vice versa. This would then result in a situation where the merger of the documents results in an out of balance transaction, and then you would receive the error: "The transactions on voucher xxxxxxx do not balance as per xx/xx/xxxx. (Company currency: -x.xx - secondary currency: x.xx)".
To resolve this issue, you have a few options: