The performance of synchronous scenarios?

Hi gurus,

We are planing to implement the integration between SAP and WMS(Warehouse management system) . The customer wants all the scenarios (Purchase Order, goods received,...) to be synchronous. Although it's good from the functional point, I think the performance may be not good.

We also need to consider the timeout isssue. For example, when the network is down or the application data is wrong, then the sender system will wait a long time or received the cause of wrong application data. If the application data is wrong, whether we should allow user to save the data in sender system? If not, they will cry if no solutions found in short time. If yes, it will cause the inconsistent data in two systems.

What do you think about this? Is the performance bad according to your experience? How to deal with the wrong application data case? Any help will be appreciated!