Asking for guidance with innovative strategies

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The 60/70 yo couple that made an offer could get only 125K, not the 140 they offered.
They have to keep their payments at 900 a month, and were planning to get an FHA loan since they don't have the 20% down for conventional.
The house has not been appraised.
How can I / how should I offer to finance the remaining 15K?--or how can I loan them the money for the down so they can get a conventional loan, and when do they pay it back?
Thinking of two IOU's of 10 and 5K, but what interest rate? I don't think they have any collaterall so there would have to be a due on sale clause, and they would have to be recorded.
How and when do they pay on those notes?
Both would have due on sale clauses (how do we write the notes?) and the propety is not to be leased or rented out.
And what happens if they default on their mortgage to the bank? Will I ever get that $15K?

What didn't I ask you folks,
What do I need to ask the buyers, what do I need to check?
When do they write (and notarize) the ious to me, before they get their mortgage or after?
Do they tell the lender the sale price is the mortgage amount, (125K, or the full amount, {maybe 140K?} )
Which is reported to the city/county/real estate/title co?
WHO reports it to the real estate commission, the city? The title co, or the lender? Or ME?

Help me work this so I can sell that house to them.
I will use a title co to close. Do I need an attorney to write the IOUs, or do you have a boilerplate one you can post for me?
We're beginning discussion of this tomorrow (Sunday) morning.

Thanks,
Ofgift
Who really needs a lot of guidance on this...