In many real estate contracts there are certain contingencies that a buyer typically must waive as a condition for closing escrow. Such contingencies are typically an inspection contingency as well as a loan contingency.With respect to the question that you have inquired about obtaining a pre-approval letter from a bank or other lender is the demonstration of the buyer to secure financing to buy the property that presumably is in contract. I always recommend that the buyer in such a situation where there is a loan contingency to buy the property not waive the loan contingency until close of escrow. At close of escrow, the buyer signs many documents to close escrow including the loan document assuming a loan is needed to buy the property.In most states there is a three (3) day period after the buyer signs the loan documents for him or her to cancel the loan.Good luck.