changes made in po after release

Hi,

  My client requirement is that he wants to get the purchase order modification done details on the print out.

Purchase order is created and is released, and now if i do changes to that p.o then in printout i need to have the "PO created date" and the P.o modified date"

I have changed the P.o now and send to vendor and now again im doing some changes on p.o and sending it back to vendor, and vendor should understand that he should ignore the 1st p.o and the current p.o is correct.

How can I do this.

Thanks

Abdul.