You could model it after trade discounts on retail: Book the full rent a Gross Rent Revenue revenue account, book the discount to a Rent Discount contra-revenue account, and the balance to a Rent Receivable asset account.
So, assume you charge $1,000/month for rent, and give them a 20% discount for the first six months. Each month they get the discount you'd book:
DR Rent Receivable $800
DR Rent Discount $200
CR Gross Rent Revenue $1000
If you give them "free rent", you'd book just DR Rent Discount/CR Gross Rent Revenue. Once the discount period is over, you'd just book DR Rent Receivable/CR Gross Rent Revenue.
Then in your income statement, you'd net the Rent Discounts account against the Gross Rent Revenue account to get a Net Rent Revenue line.