Contract are intended for use, when both parties are happy with the terms.
When one or both are unhappy, then you have problems.
You wanted them to pay Closing Costs; not because you didn't have the money, but because you wanted to bleed them for everything you could get.
If your taxes are based on the Selling Price of the house; you did yourself a dis-service because you would be ahead by paying the Closing Costs and Offering 5% less for the house.
The Sellers probably didn't read that you were paying CASH, but accepted the Closing Costs to make the deal: Then, when they read the details, they got upset and want to back out.
Now, we've got two rams, buttin heads, and no one wants to get in between them.
Which one of you is going to BLINK?