3rd Party WAN Optimization Reverse Path

Hi all

One of our customer would like to implement a 3rd Party WAN Optimization (not FortiWAN). 

An Topology is in the attachment.

Asymetric Routing is enabled and Reverse path check is disabled.

My issue is how to configure the Fortigate to allow and SNAT Traffic with a source IP from a wrong Interface. From the Session Flow I get the following error:

"SNAT mismatch policy 50 nat 1 ip 0.0.0.0, drop"

Thanks for helping in advance.