Firstly, Trading Terms aren’t accounting transactions, they are merely a reference point to when a supplier expects to be paid.
Secondly, is the transaction a single supply/purchase with scheduled payments or does the transaction have multiple supply/purchase points which has progress payments.
Thirdly, you state “I don’t have three different invoices”, well you do, just photocopy the invoice twice and then add a character (say a, b, c) to either the start or end of each invoice number, put amended dates on the two photocopies and amended the invoice values on them all. Now there is 3 invoices, The added character will held to identify where the invoice/payment schedule is at.
My only question would be, does Manager have any inbuilt restrictions on processing dates in advance, if yes, just put the duplicates in a folder for later processing, or do that anyway so that the related reports only show past dated and not future dated invoices.
As I don’t use the Supplier/Purchase Tabs, I can’t comment on how processing future dated invoices may impact the reports but to solve your issue “my payment due will never reflect the real situation and this could put my cash flow at risk” perhaps you need to either a) don’t process any invoices before their date and/or b) build a spreadsheet which would reflect the cash flow management for the project.
What you are asking for is more aligned to Project Management/Job Costing software which unfortunately Manger is not
Alternatively, you could request the supplier to provide progressive invoices based on the dates of the trading terms.