If you are not facing financial or transfer deadlines, and are under no duress, I'd tell them to walk or do the work and take away other concessions to compensate you.  

Does the seller not consider your offer because its not in his terms and time deadline?

We just asked for quality work in timely manner.I've found that quality work and people that can meet deadlines is well worth the price.

However, if you fix it before certain deadlines reduced penalties apply.

Might be a good idea to find out when students have to make a commitment to living on-campus - that's usually done mid-school year, and you'd want to market to them prior to that deadline to get interested students committed to your house prior to that so that they - and their parents - will know they have a place to live before they decline student housing for the next school year!

That said, if a contractor has a reasonable contact that contains:- Independent contractor statement- Insurance clauses- Scope of work- Payment schedule- Milestones/deadlines- Etc.I'd be okay signing it as the only contract. 

If they do not meet a agreed to dead line to pay in full you issue a eviction to protect yourself (you can always withdraw it when they pay).

So get a bid for power washing and painting / sealing.. and get that sent to them ASAP so you don't miss the 21 day deadline.  

I quickly realized that the market was entirely too hot and it would take me a lot longer than my previously estimated 3 week deadline to get off of a friends couch.

I'll be up early September, and I'll have my father go by before their deadline.