<xs:attribute name="source-port" type="xs:positiveInteger" use="optional">

            .setAllowExpression(true).setValidator(new IntRangeValidator(0, 65535, true, true))

    public Integer getAbsoluteSourcePort() {
        if (this.sourcePort == null) {
            return null;
        }
        if (this.fixedSourcePort) {
            return this.sourcePort;
        }
        final int portOffset = this.socketBindingManager.getPortOffset();
        return this.sourcePort + portOffset;
    }

[standalone@localhost:9990 /] /socket-binding-group=standard-sockets/remote-destination-outbound-socket-binding=proxy1:add(port=9090,host=127.0.0.1,source-port=0
{
    "outcome" => "success",
    "response-headers" => {"process-state" => "reload-required"}
}
[standalone@localhost:9990 /] /subsystem=modcluster/mod-cluster-config=configuration:write-attribute(name=proxies,value=[proxy1])
{
    "outcome" => "success",
    "response-headers" => {
        "operation-requires-reload" => true,
        "process-state" => "reload-required"
    }
}
[standalone@localhost:9990 /] :reload
{
    "outcome" => "success",
    "result" => undefined
}

-Djboss.socket.binding.port-offset=100

20:15:40,471 ERROR [org.jboss.modcluster] (UndertowEventHandlerAdapter - 1) MODCLUSTER000043: Failed to send INFO command to localhost/127.0.0.1:9090: Permission denied (Bind failed)