Hello,
I guess the reason why your purchase order is getting directly approved is because the person who is sending the document for approval has unlimited approval rights.
Please check user approval status in the system for it.
With respect to workflow,
The imagination that you have is perfectly fulfilled by Base Business Central. I have highlighted the steps below, please follow the same.
Step - 1 - Go to Workflows.
Step - 2 - Click on New Workflow from template.
Step - 3 - Click on Purchase Order Approval Workflow.
Step - 4 - Open the Purchase Order Approval Workflow & Click on 1st Condition shown in the screenshot below.
Step - 5 - Enter the Amount i.e., >5,000 or <5,000 i.e., Depending on the Limit which you want to set for the Purchase Order amount approval.
Step 6 - If you want a specific person only to approve above this amount then, Click on Then Response,
Click on the third line i.e., Create an approval request for record. As soon as you click on third line, you'll see few other option below
Follow the Pattern Shared below, I have selected user Vishal, you can put any user you want there (This user will be responsible for the PO above certain amount i.e., >5,000)
Click on OK.
Step 7 - Enable the Purchase Order Workflow.
This will take care of the Amount related Approval Mechanism on the Purchase order Approval Workflow for Specific user.
If you face any issue, Please share it with a screenshot for ease of understanding.