Context problem. ALL, ALLSELECTED, ...

Well, I'm not English, either.

First of all, you should use Power Query to get your data into good shape. It's easy to use PQ and it's a very powerful data mash-up engine.

Secondly, you should read on DAX because you're completely in the dark on how it really works. If you do invest some time into it, you'll save yourself hours of frustration. I promise.

-- First of all, your Calendar must be marked as a Date Table
-- in the model for this to work correctly. Secondly, you
-- don't probably know this but it's of crucial importance
-- that when you filter a measure by a whole table, you are in fact
-- filtering by THE EXPANDED TABLE version of the table in question.
-- You should never filter by a whole table unless you're asking for
-- trouble or you know precisely why you do it. When you filter,
-- please filter by individual columns only. You should also know
-- that RELATED should only be used when there is a row context
-- present, so it can be used in iterators only. CALCULATE
-- is not an iterator.

[Payable To Date] =
var __maxDate = MAX( 'Calendar'[Date] )
var _payableToDate =
	CALCULATE (
	    SUM ( InvoicePayablePart[Debt] ),
	    'Calendar'[Date] <= __maxDate
	)
return
	__payableToDate
	
[Paid To Date] =
var __maxDate = MAX( 'Calendar'[Date] )
var __paidToDate =
    CALCULATE (
        SUM ( InvoicePayedPart[Payed] ), -- change to Paid
        ALL( 'Calendar' ),
        InvoicePayedPart[PaymentDate] <= __maxDate
    )
return
	__paidToDate
	

[Outstanding Debt] = [Payable To Date] - [Paid To Date]

Best

Darek