What could we do if we are forced to sign an agreement before full payment?

We are building a home for a customer and have fulfilled our agreement in order to get a fourth payment. Although our contract states that the payments should not be held up without a legitimate cause, our customer refuses to pay the amount due unless we sign a new mediation agreement adding specific items to the contract. We estimate these items to cost around $3,000. The amount of the payment due is $47,000 and our subcontractors are waiting to get paid. Unless we get the payment, we will be put far behind in our obligations. We feel that we have no choice but to sign the new agreement. We are also afraid to finish the home once we get the check for fear of her not paying us the final payment. We want to finish the home and get this over with without the worry. What can we do?