Can a buyer lower my assigment fee after closing

Yesterday the buyer of a contract I assigned to closed on the property. I assigned him the contract for 20,000 dollars. The purchase price of the property was $170,000 dollars. I am supposed to get my money at settlement. However when the HUD-1 Statement came in, the total fees were over the purchase price. It turns out that the couple took out a credit line of 7,000 dollars which registers a second mortgage. to make a long story short, with interest it out came to a total of $11,000 dollars. Back taxes, a water liens and closing came to $6500 dollars and the Payout of the mortgage was 158,000. Also the couple said it they don't get 8,000 they would back out of the deal. They buyer agreed to this and feels that some of the additional fees should come out of my assignment fee. Can he do this. Oh yeah the current fair market value of the place rehabilatated is $360,000. It should cost no more than 15,000 to rehabilatate the place. If the buyer back out I could have sold the property for 230,000 on the market as is. There is a clause in the contract that says

"Should said Certificate reflect any other exceptions to the title unacceptable to Purchaser, Purchaser shall notify the Seller in writing of any defects within fifteen (15) days (the title review period) and the Seller shall have a reasonable time (but not more than 25 days) in which to make the title good and marketable or insurable, and shall use due diligence in an effort to do so. If after using due diligence the Seller is unable to make the title acceptable to Purchaser within such reasonable time, it shall be the option of the Purchaser either to accept the title in its existing condition with no further obligation on the part of the Seller to correct any defect, or to cancel this Agreement. If this Agreement is thus canceled, all money paid by the Purchaser to the Seller upon the execution of this Agreement or upon any extension shall be returned to the Purchaser, and this Agreement shall terminate without further obligation of either party to the other. If title is acceptable to Purchaser, the closing shall occur within fifteen (15) days after."

First of all it was passed fifteen days when he knew about the defects and he didn't notify anybody in writng. So I think legal he waive his right for the seller to perform due dilligence. but there is another clause it the contract that states this "Seller represents that as of the Closing Date (a) there will be no liens, assessments, or security interests against the Property which will not be satisfied out of the sales proceeds unless securing payment of any loans assumed by Purchaser and (b) assumed loans will not be in default. If any representation in this contract is untrue on the Closing Date, this contract may be terminated by Purchaser and the earnest money will be refunded to Purchaser. All representations contained in this contract will survive closing. What does this mean for me practically and how am I protected as the assigner legally in theory . P.S. there are no clauses in the assignment agreement that have any contingency of going over price. The contract states that that I get my money at settlement for a contract he purchase for 20,000 for a house to be purchased at 170,000.

Does the buyer excepting the new terms of the homeowner telling him to increase his purchase price or he walks from the deal void the 170,000 dollar purchase price he sign with us. And I legally binded to make up the back difference of the purchase price and and What can I do to protect myself from buyer getting funky with my assignment fee.

P.S. there are no clauses in the assignment agreement that have any contingency about going over price. The contract states that that we get our money at settlement for a contract he purchase for 20,000 for a house to be purchased at 170,000.