You will be able to enter details on the Installment Sale, and include the 6.5K as a Sales Expense.
In the Rental section, under Property Profile, indicate that you 'sold or disposed of this property' and the date.
Then in Sale of Property/Depreciation, EDIT your rental home Asset and indicate it was Sold and the Date. TurboTax will give you a Prior Depreciation amount (screenshot).
Say NO to '..included in sale of your main home?'.
On the Sales Information screen, you can include in Sales Expenses the cost of the new AC unit, plus any other expenses (screenshot).
YES you '..used this property as Personal Residence after May 6, 1997?'.
The following screen asks 'Do you want to report this sale using the installment method?' If a mortgage was paid off, enter that amount. If not, Continue.
Enter Principal and Interest received in 2018. TurboTax reports the Gain/Loss on Sec. 1250 property and the Installment Sale Income for 2018 (screenshot).
You will see the Installment Sale Worksheet in your return.
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