I am selling a house... in order to ensure buyer actually qualifies for the mortgage, I typically  ask that he qualifies with my mortgage lender as well (mortgage lender is a friend of mine and I have asked him to please check in detail all income documents and bank statement before pre-qualifing my buyers)..

Buyer is a first time buyer and is a bit irrational.... does not want to do it.... he will not sign my counteroffer asking him to make this step. 

I really do not want to hold up the house for a month or 2 to then find out buyer does not qualify

I thought to stop pushing that he qualifies also with my friend, but to include a clause in the contract that within 15 days from the bringing date he needs to provide a document  stating he is conditionally approved by an underwriter. 

did anyone ever do something similar? can you share experiences? how do you ensure a buyer is qualified?

Usually before I even start to negotiate with a buyer I make sure they are approved with a lender and then once the contract is executed, within the contract it states that they have X amount of time to get fully approved in order for the deal to keep moving forward and them to not lose their EM. 

what is the X amount you useto get fully approved? 

Is 35 days reasonable?

Offer some incentive for him to get pre-qualified from your guy, like maybe you offer to pay the appraisal fee if the buyer did that.