Different SNAT for each member in the pool

Thank You so much Stefan,

The proposed iRule looks right and good but I cannot approve it as working solution,
 I really sorry for that but in our case we use exactly similar iRule:

when LB_SELECTED {
   if { [IP::addr [LB::server addr] equals 193.142.151.1] } {
     snat 172.16.50.1
   }
   if { [IP::addr [LB::server addr] equals 193.142.151.2] } {
     snat 172.16.50.5
  }
   if { [IP::addr [LB::server addr] equals 193.142.151.3] } {
     snat 172.16.50.6
  }
}

(SNAT Pool includes these three Nodes: 172.16.50.1, 172.16.50.5, 172.16.50.6)

And in the FW LOG catched just after F5 we still see Requests from Src:172.16.50.5(6) to Dst:193.142.151.1 , and I can't understand this.

According the iRule such packets are impossible - When Dst:193.142.151.1 the only source IP must be 172.16.50.1   

 Respectfully,

Evgeni V.