max as a filter

Without going into whether there are any other issues with this approach, to get around the problem you have raised you'd just need to put it into a variable:

Sales = var filterDate = max(Query1[InvoicePeriod])
RETURN
CALCULATE(
     sum(Query1[SalesValue]);
     Query1[InvoicePeriod] = filterDate
)

You'll also need to specify what the filter condition is.  I've assumed you want records based on that date.  However you could use <= if you wanted to get all records on or before.

I'd suggest creating a date table and having the slicer be based on that date table.  Then, for variable filterdate, do the max on the DateTime instead.