Rent to buy question?

I will have lived in my house for two years in October. I plan on renewing my lease for one year again, however I have been considering talking to my landlord about possibly buying my house. I have applied for a mortgage, but because of bad credit, I got denied.
The value of my house is approximately $150,000. Over the past two years, I have paid $32,400 in rent. I’m not 100% sure, but I have a feeling he might be willing to work something out, but he works in real estate, so I want to make sure I dont get ripped off. I would like to take some of the previous rent paid and use that as a “down payment,” which might help my chances to get a mortgage.
What would be a good percentage of rent paid to start as a bargaining point?
Does anyone that has experienced a similar situation have any advice?