You can either reduce the rent for that particular month:
Tenant Manager => Tenancies => Hover over the tenant's name to bring up the cog icon => Select "Change Rent".
On this screen, change the "End Date" against the existing line to be the last day before the month of the reduced rent. Click on the + symbol on the right to add a second line. Enter the details of the reduced rate. Click on the + symbol again and then enter the details for the normal rate to resume. For example:
£750 Monthly 01/01/2015 31/08/2015 1 of the current month
£600 Monthly 01/09/2015 30/09/2015 1 of the current month
£750 Monthly 01/10/2015 31/12/2016, etc.
Alternatively, you can record that you have received the full amount of rent from your tenant but then issue a Credit Note => Refund for the amount that the tenant retained:
Tenant Manager => Invoices => Add Credit Note => Enter Credit Note information => Save => Add Refund.