Refund of advance cst paid

Dear sir,

One of our client  , a granties (wood) suppying company, they transproting timber from one state to their buyers in another state. At the time of transporting the timber , our company issuing bill to the buyer company , for the weight of timber transporting from supplier location and paying advance tax at check post for the bill amount. But at the buyer's part , they are making payment only for the weight of timber what they are getting at the factory site. They are calculating the bill amount based on the quantity they are receiving at factory and cst also to be calculated based on that. So the buyer issuing C form based on the quantity what they received. please guide what to do in our company's books . our company is paying advance CST more than what  actually collected from buyer