(Love your name)
The seller signed the orginal offer the buyer sent with the original terms including the closing costs listed in the PSA. The deal is fully accepted and deemed an agreement.....until your buyer changes something. Once he makes that change all parties have to agree, therefore, until it's signed around again, it's not an agreement.
The seller can accept or deny the new terms. In other words, I do believe the seller can refuse, it's ultimately up to the seller as it's their credit and defiency loan amount in negotiations.
Looking at your market stats it appears that you have a downward trending market in terms of values, is that correct? Balanced from the inventory standpoint, or is it seller's market?
Your buyer needs to know the type of market. If months of inventory is below three months (which might be the case here with 450 homes for sale and 260 homes recently sold) then your buyers need to be aware that the sellers can "make the rules".
Buyer and seller agreed on a sales price, not buyer is not.
Yes, if you change the terms outside of the agreement. It is the seller who is signing the RPA and then submitting to the bank. It needs all hands shaking to make it happen... Seller, Buyer and Bank. They want it to go smooth and the bank has already agreed to specific terms. Now they want it done and over. The bank can make it very difficult and they generally want a clean fast close once agreement has been made. But... that is the nature of the beast.