Service tax rules for discount received after payment

Kindly give suggestion to below mention situation with service tax rules / section no / Case Law

Mr. A paid Rs. 110.30/- (cost +St) to Mr. B on 30-06-2011 for some service rendered by Mr. B There after on 20-07-2011 Mr. B gave discount of 25% to Mr. A for the above payment.

Mr. A is eligible the discount from Mr. B for the Cost of Rs. 25 or the Cost + ST 27.58 (25 + 2.58)

Mr. A is eligible the discount from Mr. B for Rs. 25 or Rs. 27.58