Do I Have To Give The Diffrence Back To The Buyer

I Was Contacted By This Person When He Contacted Me About Buying My Equipment And I Told Him My Bottom Dollar Was $35000.00 USD. He Sent Me A Check From The UK. For $35000.00 Pounds Which Is A Little More Than $54000.00 USD. He Told Me To Put It In My Bank, Which I Did Into A New Acct. Now He's Telling Me To Send The Diffrence To His Shipping Company When The Funds Clear My Bank. Do I Leaglly Have To Send Them Anything Or Can I Keep It.