Can a manager at my corporate client approve a trip before their employee is allowed to book it?

Yes, approval before booking is one of the standard pieces of a corporate travel setup, and most portals can be configured so an employee searches and chooses a trip but cannot confirm it on their own. What the employee does is build the trip and send it as a request. The named approver, usually their line manager or a travel desk, gets a notification with the full details and the cost, and can approve it or send it back with a reason. Only once it is approved does the booking go through to the supplier.

The rules around who approves what are usually yours to set with each corporate client. Some companies want every trip approved, others only want to see trips that break their own travel policy, such as a higher cabin class or a hotel above their nightly limit. Several companies want two approvers for bigger trips, one for the department and one for finance. Whether all of those options are available to you depends on the plan you are on and how the portal has been set up for that client, so it is worth agreeing the rules with them in writing first.

The honest limit is time. An unconfirmed trip is not a held booking, and airfares in particular can move or sell out while an approver is busy. Some fares can be held for a short period and some cannot be held at all, and that is decided by the airline or the supplier, not by the booking system. So approvals that sit for a day or two often come back with the price changed, and the employee has to pick again.

Before you switch it on, sit down with each corporate client and agree who the approvers are, what needs approval and how quickly they will respond, and check with your main suppliers which of their fares can actually be held while that happens.