Current UPB of note: $21,544.08I sold 90 payments of $300.15 for $20,000.How do I record this???

Although FHA loans need little down payment, there may be a higher interest rate.

If your accountant has used the strait line method there is nothing to worry about, but also nothing to "capture" on sale.

The dollar you spend on ADU will be sufficient for the down payment of SFH in south or midwest.

How do you all structure the monthly payment if a tenant cancels the lease midway through the month? 

You could use the money towards a down payment on a new property.

Rental assistance/payments will begin the date the property passes the inspection.There are landlords that do not like the Section 8 program or want to be involved with it, either due to a bad tenant experience (which can happen in or out of the program) or simply from stereotype or ignorance of the program. 

Refinancing can allow you to pull out equity, providing you with additional funds to use for the down payment on a new property.Look into financing options for purchasing a duplex.

Rarely covered in the version that's turned over, but important edits will set expectations, payment schedules, and includes specs/allowances.

Check with your lender what your new payment would be on the existing and what that would allow you to afford on your next purchase.You can still do an owner-occupied loan up to 4 units but if you're using your FHA/VA loan you'll have to go conventional 5-10% down.