Using an option as debt

[Philip Klinck]

So I want to buy a house by giving an option against another house. If I dont know what the exact selling price is how can I be sure what to write the option up for? 

Example:

Purchasing a house for $50k with an option against a house I currently own worth around $130k. 

The way I understand options is that I could give the seller of the $50k house an option to buy my house for $80k and when it sold they would get $50k. But, what if it sells for $125k or $135k? Then the seller of the $50k house will either get underpaid or overpaid. I want it to be exactly $50k and I dont know when the house is going to sell or for exactly what price. 

I realize I could give them a 1st mortgage but Id rather keep things simple and just write up a contract that gets recorded.