Question about renting and conditional sale?

I rented a home where the married couple seperated the female has maintained and resided at a different address for the past 4-5 months. The husband also a resident in the home I rented (Basement), is the landlord. We made an agreement that I rent the home and all rent is applied to purchasing the home. Also several thousand dollars was required to make the home inhabitable (video taped evidence of that). The agreement for work made between the landlord and myself is that in the event for ANY reason, I can't purchase the home the costs will be paid to me at current contractors rates (3 estimates form certified contractors) and the costs must be recovered before I vacate the home. Both parties are on the title and mortgage, as well as the landlords Mother is the primary on the Mortgage and agrees to this. I understand the wife would have to sign off or agree to the sale of the property. My question is: Is it legal to enter into a conditional contract like this with just one of the homeowners?