You receive the error: “The transactions on voucher xxxxxxx do not balance as per xx/xx/xxxx. (Company currency: -x.xx - secondary currency: x.xx)” when attempting to post an invoice proposal in Project management and accounting.

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:

1. Uniquely define the number sequences with a character segment specific to each document type.
2. Use the same number sequence for the document types.
3. You could also resolve the error by bumping up the next number to something that would not be a duplicate.  However, this would be a temporary solution as you would likely run into the error in the future on other documents.