Frank -- not sure if you tried this or not. Within the Join set up (where you specify one field in each table to match up), choose the 'Create Join Calculation' option at the bottom of the list of fields from Cities and use the calculation INT([municipality_id]). This should make it a whole number. Then set up the right side to look at the municipalities_id field as normal.
That worked! I did not realise it was that easy
Thank you so much!