Clarification reg. service tax exemption

Dear all,

I'm an article assistant in chennai and i need a clarification reg. Servica tax.
I came to an audit where the cleint repais the ships that comes to chennai port and no where else they do this , only in chennai harbour.
They have Indian customers and foreign customers too. 
Now they are in an assumption that they can take ST exemption if they receive payment from the foriegn customers in USD.
please clarify reg this issue. Rule 6A of service tax says NO
Please clarifiy as soon as possible with relevant provisions / Case laws.