And it helps me retain clients after I help them buy a property which increase the chances of them listing with me in a few years.

Hiring professional management though will make it truly hands off and passive, while still retaining the positive upside of increased rental income.

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Suggest to offer the appraiser a retainer and/or a service contract by the hour.

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The reason I mention the not cutting of grass for the guy with the rat problem - the house is sitting about 30' high above the street, on top of a huge retaining wall.

-are retainers involved?

As with anything you purchase for your business or personal, always negotiate what is yours or what rights you have retain items and what you are paying for. 

I can't see any mechanism where you could retain your FHA mtg, doing a partial payoff either, but maybe there is a way to do it.

Look for phrases like lender "shall forgive the deficiency/balance" "shall not pursue the balance of the debt" "shall waive the deficiency /balance" "shall consider the $ received as full satisfaction of..." etc.NOT "lender shall lender release the mortgage for the $ received, but retain the right to puruse...."