Route local network through VPN, assign multiple external IPs

I have a network like this:

And I need each Black box to operate from a sepcific IP of Server B.
Both server run Debian 9. I can only change network settings (IP/Gatway etc) but nothing else on those blackboxes. How do I do this?

Something around this? Setting gateway of the boxes to 192.168.1.100

Server A

iptables -A FORWARD -s 192.168.1.0/24 -o tun0 -j ACCEPT  
iptables -t nat -A POSTROUTING -s 192.168.1.101  -j SNAT --to 10.8.0.9  
iptables -t nat -A POSTROUTING -s 192.168.1.102  -j SNAT --to 10.8.0.13  
iptables -t nat -A POSTROUTING -s 192.168.1.103  -j SNAT --to 10.8.0.18  
iptables -t nat -A POSTROUTING -s 192.168.1.104  -j SNAT --to 10.8.0.22  

Server B

iptables -A FORWARD -s 10.8.0.0/24 -o eth0 -j ACCEPT  
iptables -t nat -A POSTROUTING -s 10.8.0.9  -j SNAT --to xxx.xxx.xxx.xxx
iptables -t nat -A POSTROUTING -s 10.8.0.13  -j SNAT --to yyy.yyy.yyy.yyy    

Solution 1:

I can only change network settings (IP/Gatway etc) but nothing else.

Then it is not possible. Your question doesn't specify if you need 2-way connections (i.e. having "black boxes" accessible from the Internet) or not, but anyways this would require at least iptables for SNAT and advanced policy-based routing with iproute2.

Added after original question was edited:

Yes, something like this could work. But that is strongly dependent on your OpenVPN settings. Those local IPs from the VPN subnetwork (10.8.0.0/24) should be set up on your Server A and routed through VPN tunnel. Also do not forget to allow related packets coming back. This is usually done with something like this rule:

iptables -A FORWARD -i tun0 -m state --state RELATED,ESTABLISHED -j ACCEPT

On Server A. And the respective rule for -i eth0 on Server B. And you have to set Server A as default gateway for "black boxes".

But instead of doing overhead of double SNAT, you might also consider setting up your OpenVPN to allow to route 192.168.1.0/24 subnetwork directly to and from Server B. There are multiple tutorials on the Internet on this configuration.