This is an unusual structure but not in and of itself prohibited. The deal must address the occupancy prior to closing (I assume your tenant wants to remain in possession). What is the purpose of this extended closing period? I assume it is to permit the tenant to accumulate the necessary downpayment, but bear in mind that you (both) risk market exposure, but yours is greater. The tenant has a stop loss at 10% downward prices, as the typical contract deposit of 10% is walked away from typically when prices drop below that. You, on the other hand, have no such protection, and if market conditions increase 15% (unlikely, but possible) then you will have the lesser end of the bargain. Proceed carefully here.