How can I convert Inward purchase order(sale order) to Sale?

First you have to create inward purchase order(Transaction > Inward Purchase Order) then you can convert it into sale. To convert inward purchase
order directly in sale:

• Go to Reports > Transaction Register > Inward Purchase Order Register.
• You can search Inward Purchase Order by Ledger, Date, Sales man, Billing classification, Billing Group,and Tax group.
• It you don’t remember anything from above options, then you can directly click on Show button.
• The Inward purchase order entries will show.
• click on line which you want to convert to sale.
• Then in toolbar you can see the Convert to Sale button.
• click on Convert to Sale button.
• Now sale window is open (if you want to change data you can change in sale window)
• Click on OK button.