You'll have to have a paper trail proving where you deposited the downpayment. If you gave the money to the seller already, you'll have to prove that money was given and no you won't have to have the money in the bank if the underwriter is satisfied with the documentation you provided. Usually, you don't give money to the seller directly. It's usually deposited with an escrow company to hold on your behalf. You will also have to show where the closing costs are coming from. With an FHA loan, the seller can pay most of those cost but again you'll have to have documentation (the purchase agreement) proving seller is paying them.
I hope that helps,
Real Estate and Mortgage Broker