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Most often, I've heard of such a document being presented and signed from agents who offer: "We guarantee to sell your home or we'll buy it." In this case, there is a very structured price reduction process to bring the price to, let's say, 70% of the listing price within 120 days. But obviously that takes a motivated seller with a good amount of equity in the property.
Hope that helps.
When listing i always tell them what it is worth in todays market, whet they should ask and what i think they will get, most sellers want to increase the asking price, my respoinse is we can try, but in 2 weeks if we have a bad response or no showings, you are going to want to reduce to where my bpo is telling us we should be. putting that in writing is better than a set 1% lowering each month per say.
good luck and please let me know how you make out.