given the way that you have stated your problem, there is no solution. This is not a QlikView problem but a fundamental data problem. That is, you would not be able to do this with any product, because your tables have insufficient information.
The key question for you to think about is this: would you be able to manually combine these tables using a spreadsheet? At the moment, you cannot, because your Invoice table only contains date information, and your Sales table has no date information - they are completely different information, with nothing in common that enables you to link them together.
I'm guessing that Sales is meant to be individual line items on the invoices. If this is so, then you need to have a way to uniquely identify each Invoice (e.g. an Invoice Number) and make sure that is part of the Invoice table; and for each line item in the Sales table, you need a reference to the Invoice that it belongs to (i.e. add a column to contain its Invoice identifier).
Hope this helps,