Hi Raut,
There is feature of 'Purchasing policies' that you can use to skip workflow approval. This is standard functionality. In order to access the feature, Proc & Sourcing > setup > policies > purchasing policies.
Create a new policy, then go to rule "Reapproval rule for purchase orders". Create a new policy rule and select fields that you want to consider the reapproval rule.
Later in your workflow add condition "require purchase order reapproval" and you should be all set. you just need to make sure your workflow is setup right and you have setup your fields correct in purchase order policies.
Take a look at blog below that explains the setup. Hope this helps.
gfeaxblog.wordpress.com/.../
Thanks,
Satish Panwar
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