Are you picking up the $2,700 as income? You can not have it both ways, i.e., take a deduction and not include the income used to generate that deduction. Without the income, you have no basis. That is where you need to start with this.
After that, if it was truly a rent with option to buy, the option was exercised in 2018 and you would put the $2,700 on the depreciation schedule for 2018.