1031 and an option to purchase in future

Have done multiple 1031s over the years but the question came up on a seller that is agreeable to terms of a future for sale while the buyer is looking to sell another property and porting the funds at a future date within the next 18 months after a sale is completed.  

variables would be 10 percent down for the option and 2500 a month rent/paydown towards the purchase amount.

can not find a reason why it could not be ported exempt funds except maybe not for the total amount (settled price minus down minus paydown)

thanks