This is a very hypothetical query and such query should not be answered because filing reply without perusing terms and conditions of contract executed between 'A' and 'B' will not be complete and concrete. My premonition is that it is just planning and contract is still to be executed.However, since Sh.Rajagopalan Ranganathan, Sir, has invited comments so I would like to opine as under :-
(1) In this situation both 'A' and 'B' have provided taxable services. 'A' has provided taxable service under the category of 'Renting of Immovable Property' and 'B' has provided service under the category of 'Recovery Agent Services under Section 65(105)(zzzl). (Now Other Than Negative) or under the category of BAS (now other than negative) depending upon the nature of contract/agreement. Proceeds to be received by A from B is nothing else but rent of commercial property. If A has recovered rent through B it does not alter the classification of service provided by A. If 'A' is a body corporate or firm, then 'B's services will fall under the Service Tax net.
Since B provides service for and on behalf of A, so B's services clearly fall under erstwhile B.A.S. (see definition of Commission Agent Explanation- (a)(iii) to BAS. (Now we can say 'Other Than Negative)
Now it depends upon the nature of the agreement so as to determine where B's services are classifiable under Recovery Agent Services or BAS. It is certain that both services are taxable. Old accounting code still exist not just for statistical purpose but also for clarity the concept of any service.