I want to see an increase in earnest money equal to the funds coming to closing - i.e. if it is FHA, they would need earnest money of the 3.5 % - and it would not be refundable if the buyer does not close. Lenders don't like to lock rates that far out and this could be an issue, but as long as the inspection and appraisal are done within a normal time frame, say 4 weeks, then it can sit under contract for another 6-8 weeks. For me, it is all about making sure my seller is not harmed by taking the home off the market - and putting more of the buyer's money at risk for non-performance usually helps. Also, make sure you continue to market the property for back-up offers.
The other way to go about it would be to do a Right of First Refusal which would put the buyer in first position, but allow the seller to entertain other offers, givng the buyer the opportunity to drop the contingency if the seller receives a competing offer. Both ways work!