Past years I was able to enter this amount and get a deduction for that.
I can assure you with 100% certainty that you have never been asked for the rent you charge. You have only been asked to enter the amount of rental income you actually received in the tax year. You also did *NOT* get a deduction for charging below FMRV. In fact, you lost money for that. (Well, not yet. But you will when you sell the property).
When you rent below FMRV, your carry over losses are just flat out not allowed. You lose them, permanently and forever.
