I checked the source address and it seems to have enough balance:
http://mastercoin-explorer.com/addresses/1HRE7U9XNPD8kJBCwm5Q1VAepz25GBXnVk

Here's the details for this trade:
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Seller: buymastercoin
Buyer: Pouncer

Amount Mastercoins: 200
Price per Mastercoin: 0.0181 BTC
Total price: 3.62 BTC

Mastercoin source address: 1HRE7U9XNPD8kJBCwm5Q1VAepz25GBXnVk
Mastercoin destination address: 1MSCXpZHRR9DJpGcnQT1Nr1VDcL25fXfbP

Bitcoin escrow address: 1CXrw63tjXYbebZaNhMUhHeUcKRitrxsSK
Bitcoin receiving address: 14gF4YGKxsPx7r5vC6FLTPP9MBTNvHHCyo
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Pouncer, please send 3.62 BTC to the escrow address 1CXrw63tjXYbebZaNhMUhHeUcKRitrxsSK
buymastercoin, you can send send the Mastercoins once you see incoming funds: https://blockchain.info/address/1CXrw63tjXYbebZaNhMUhHeUcKRitrxsSK