Merchant account

I have a small antique clock buisness and opened a merchants accout so I can accept credit cards. Things were going great untill I had a purchase order from Nigeria. I thought this could be a scam so I processed the customers account number and waited 5 days for the funds to be cleared in my account. Then I was contacted by my bank that has my merchants account and was told that this credit card could be fraudulent and I would be responsible for a refund to them if it was fraud. They also want me to fax several past invoices from months ago and that I may have to return the funds from those sale also.
My questions is. After I have recieved funds from my merchant account and have already shipped my items to the customer will I have to repay my bank for a fraudulent card that my merchants account bank has accepted. If the card was no good shouild'nt they know and not fund the transaction?
Dennis