Preserve source port through NAT


I have a remote PRTG QoS (round trip) probe which I imagine was created to run between edge devices
The problem is that I need to use it between devices that sit behind firewalls, and these firewalls NAT outgoing traffic

My PRTG Network Monitor isn't receiving any responses  
I have checked firewall and NAT rules and both seem  properly configured
NMAP can detect the remote probe if I specify the following options
-sU -p 50000 -g 50000
I therefore assume that the probe wants to respond to me over UDP port 50000

The problem is that, I presume because of NAT, my packets don't have a source UDP port of 50000

Is there a way to ensure that the source port of an outgoing packet is preserved ?
We have Cisco ASA 5505 router at our end and the datacenter has a (Debian) Vyatta firewall

Any ideas ?
Yann ShukorOwnerAsked:
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On Cisco router command is:
ip nat inside source static udp 50000 50000 extendable

(if port 50000 on address is  triggered - link will be established to port 50000 on and vice versa)

Extendable is needed in case that you want create port trigger - where, for example, port 50000 is forwarded to host1, and port 50001 is forwarded to host2. If all static bindings are forwarded to the same host suffix extendable is not needed.

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