GST Liability on Disputed Rent income

Due to lack of Rent contract agreement  with the Lessee the disputed Amount of rent was offered as Income in the Previous years 2yrs. However no invoice was raised for it.

Query :

1. are we liable to pay gst for the previous 2 yrs. 
2. Can we avoid offering it as accrued income in the current year on the grounds that the amount is disputed and no contractual agreement is signed.