Pool member FQDN on server side, Virtual server name on client side web browser

The above will replace the Host header on ingress and the Location header on egress (if a redirect). If the client is still getting sent to the internal URL, then it's very likely that the URL is being presented to the client somewhere other than a redirect header. Given that this is SAP, there's a fair chance that JavaScript may be involved, or at the very least document objects (images, CSS, etc.) are referencing the internal URL. The wireshark capture will tell you for sure, and the cure can be as simple as a STREAM profile iRule or as complex as a ProxyPass implementation.