It does not sound like a good deal to me but you have not given many details.
Before making a decision: I would contact a lawyer and I would also recommend you check the current market price for your condo, you can do this by looking here and also via the NY times and REBNY or more accurately by having a few agents come by and bring you some comparables, while reviewing your condo ,let them tell you their opinion of it's current value.
Please feel free to drop me an email so I can let you know what brokerage I have decided to associate myself with..
Since you are risking that the price will go up and you will lose money (have to sell below FMV), you need to be paid for that risk. What the price should be I don't know. What is the condo worth now?
Hard to say, I would really need more info...Please give us more details
Are you saying they want to rent with an option to buy?
Are they giving you a downpayment now?
Do you want to have the rental income?
The way things have been in recent years the value of property keeps going up...