if we have a link(300M) runing on interface 1 GIG;can we lie to the Interface and declare the Bandwidth as 1048576 as well as lie to EIGRP also?if so;it will work without any problems?
Wether this works "without any problems" depends on your requirements. The link will get a metric smaller than the actual bandwidth. Any 2 Mbit line would be used instead because of the better metric.
So it depends on your requirements and topology.
Also be aware, that "bandwidth" is also used in QoS policies. Therefore I would recommend to use the real 300 Mbps as interface bandwidth and modify delay (the second EIGRP metric value in use per default) to make the route less preferable.
<1-16777215> Throughput delay (tens of microseconds)
Hope this helps!