To issue a first partial rent, that is a rent that does not relate to a full month, you have multiple options:

1. you can specify it in the Tenancy creation form. If your tenancy starts at a date which is not the 1st, the site will present you with the First invoice section. You can later find this section in Tenancies > Additional Information tab.
2. You can add a revenue (type Rent) manually in the Finance page.
   If the rent has been already created in the Finances section, you can modify it, changing the dates and the rent amount.