In a real estate contract all things are negotiable. Sellers are under no obligation to pay buyer's closing costs, so if they choose to put a restriction on when and how they will pay them, that is all part of the negotiation. If their lender can offer you as good a deal as your lender, then it is a win-win for both of you. If their lender is higher, bring them your good faith estimate from your lender and see if they will match it. Since loan terms and costs are also negotiable, you might find that you are, again, in a great position to prosper from this arrangement.
Ask you realtor to help you negotiate these items. That is part of the value they should bring to the table in representing you as a Buyer's Agent. It is part of what I do to insure that my clients get a great deal on a great house with great terms. Dare to Dream.
Real Estate Consultant
RE/MAX Palos Verdes Realty