Hi! Here's a few things to consider:
1) You could write-up an option in the contract which would give you a future right to purchase the home. It would obligate the seller to sell, but only if you decided that you wanted to buy the property. Typically, you would provide some form of consideration ($$) for this right. In this case you would set the price and date (or time period) when you write up the option. Potential upside: if house appreciated and you wanted it and could buy it. Potential downside: the house depreciates or don't want it or cannot buy it, then you'd lose the $$ you provided for the option.
2) You could write up a "first right of refusal" which would give you the first right to buy the property when the seller sells. This basically gives you the right to buy the property before anyone else. The terms are not set when you write this (e.g. date or price), but those would be set when the time came. Based on your question and wanting something fair for both, this may be a path to explore. Potential upside: if the house depreciates, then you'd be albe to get a better price later if you still wanted and could afford the house. Potential downside: If the house appreciated, you would have to pay more, or you may not be able to afford it. Again, there is some sort of consideration (typically $) to have this right. Again, the right would be yours to decide if you wanted to exercise it.
3) The third is a "Lease to Purchase" - here you provide some consideration (typically $) as a downpayment and there is a specific date (or period) which you have to purchase the property. Lots of different things to watch out for (e.g. does any of your lease payments count toward the purchase price, is the price fixed in the future or is there some sort of interest rate assumption). Potential upside: if the house appreciates, you've locked in a lower price if you still want the house and can afford to purchase it. Potential downside: If you house depreciates or you don't want it or can't afford it - you lose the consideration.
You should carefully review whatever terms you negotiate for your situation and you may want to consult with a real estate professional and/or real estate attorney to ensure you fully understand what you're getting into and what the potential risks are for you.