In this case, suppose OEM manufacturer is 'A' . Distributor is 'B' and 'Event Running Company is 'C'. In the exhibition to be organised by C, B would display the goods (manufactured by A) on behalf of A for which B would pay exhibition charges to A (including applicable taxes). Thereafter, A would reimburse 50% of the exhibition cost to B. This can be termed as taxable service provided under the category of erstwhile BAS specified under Section 65 (105)(zzb) (Explanation (a)) (Now under the category of 'Other Than Negative List').
Now as rightly advised by Sh.Akash Deep, if B recovers the amount of the exhibition cost whatever he has paid to C (i.e. notmore than he paid to C in any situation) from A, there would be no consideration. In other words, service would be provided without any consideration. Hence no service tax chargeable.
Thus the question of payment of Service Tax on the amount to be reimbursed or resorting to Section 67 read with Rule 5 of Valuations Rules does not arise.
In this way I endorse the views of Sh.Akash Deep.