Remove the Row context while keeping Filter context

Try this for Internal Transactions, it will match the Laboratories example, but NOT match the Engineering as I don't see the data for Engineering + Cost Personnel in your example.

Internal Transactions = 
var _selPL = SELECTEDVALUE(TableTransactions[Product Line])
var _selPLRow = SELECTEDVALUE(TableTransactions[P&L_Row])

RETURN 
CALCULATE(
	SUM('TableTransactions'[Amount]),
    FILTER( ALLSELECTED(TableTransactions), TableTransactions[Product Line] = _selPL 
                    && TableTransactions[P&L_Row] = _selPLRow
                    && CONTAINSSTRING(TableTransactions[Company], "Comp")
                    && CONTAINSSTRING(TableTransactions[Counterpart], "Comp")
    ) 
)

For the Internal Eliminations,

* Are you trying to show the same value for Costs* rows as Internal Transactions value?
* For Revenue Services rows, are you just negating this above calculated sum of all costs rows value?

* For the rest of the rows, you are showing as zero... Kind of not clear!

Tried some DAX to get what you need ... 

Internal Eliminations = 
var _selPL = SELECTEDVALUE(TableTransactions[Product Line])
var _selPLRow = SELECTEDVALUE(TableTransactions[P&L_Row])
var _selCompany = SELECTEDVALUE(TableTransactions[Company])
var _selCounterpart = SELECTEDVALUE(TableTransactions[Counterpart])

var _Tmp1 = ABS( CALCULATE(
	SUM('TableTransactions'[Amount]),
    FILTER( ALL(TableTransactions), TableTransactions[Product Line] = _selPL
                                    && CONTAINSSTRING(TableTransactions[P&L_Row], "Costs")
    ) 
))

RETURN SWITCH (TRUE()
            , CONTAINSSTRING(_selPLRow, "Costs"), [Internal Transactions]
            , CONTAINSSTRING(_selPLRow, "Revenues Services"), _Tmp1 
        , 0)

Hope this helps or tune to your requirements!