DAX and table structure/relationships advice

Hi

I'm building a portfolio reporting tool which monitors financial spend on projects. Part of my data model includes the three tables shown and described below:

Project details - master data of project details i.e. ID, name, finance system codes. Each row represents a single project.

Summary - high level finance data for each project split by FY and funding source.

Budget - budget for each project split by FY and funding source.

Current active relationships:

Summary to Project details based on Project ID (Single, Many-to-1)

Budget to Project details based on Project ID (Single, Many-to-1)

I want to create a column in the summary table which calculates the project's budget variance = Budget - (Actual+Forecast). Actual+Forecast is in the summary table, but Budget is in the budget table. [NOTE: you will see columns called variance already in the summary table but these are based on an approval value not the budget value). 

I tried creating various measures and columns, but I think because the relationship both tables have to project details is only based on project ID the numbers are incorrect i.e. I don't just need the project's total budget, I need the budget for each specific FY and funding source.  

I also tried creating a unique budget ID in the summary and budget tables as a string concatenation of project ID + funding source + FY and then creating a relationship between the two tables, but Power BI wouldn't allow the relationship to be active as it would create ambiguity.

Am I able to achieve what I need with DAX formulas? Or am I forced to need to restructure the project details table?

Thanks in advance and sorry for the long post!

Rachel