You could block the days they are not paying for and unblock them when they message you that they are ready to pay. However you are taking ALL the risk that they will not follow through with a reservation for those days later on. There are work-arounds for everything that AirBnb puts in place to protect the host from losses, but the host takes ALL the risk if you use a work-around like that.
Personally, I think if they will be able to pay later, then they can pay now. They are using a credit card after all. If they are using a debit card or prepaid credit card because they can't get a real credit card, then they are not a good bet for paying the rest later anyway.
So, I wouldn't encourage them. I would just say, that's the way AirBnb works, sorry.