Hi Chris,
I have the same problem as you. I am trying to create a report that would show all of the invoice for a project follow by the next column showing the amount paid but I am also finding it difficult. I have managed to almost get it but what is throwing it off are some AR reversal invoices. Below are the tables I used to make the relationship along with the generic inquiry. Hope someone can help as Acumatica should have an easy way to track paid to date for projects.

Hi @ChrisWeidner were you ever able to resolve your issue? Thank you!
@Chris Hackett , I created a work around. I have a Billings/Cost Recap (B2D & C2D, by project (as of date)). Then I manually pull Paid to Date (with As of Date) via pivot table into Excel. Once both reports are in excel, I use a vlookup to combine into a single report (for over/under purposes). The Over/Unders need to be as of date (EOM), because this is ultimately what we use to tie out our job costs to our general ledger. For Paid to date I use a formula, then create a pivot table to combine Paid to Date by project by FinPeriod. Long work around, but it still is quicker than what was previously being done.
(This formula may not work for all use cases. We only use AR for Invoices & AR credit memos)
=IIF([ARAdjust.AdjdDocType]='INV',[ARAdjust.CuryAdjdAmt],[ARAdjust.CuryAdjdAmt]*-1)
Thank you for sharing your solution @ChrisWeidner !