The feature called Double validation or Multilevel approval for purchase orders in Odoo support double validation and request managers to approve orders above a minimum amount.

Purchases → Configurations → Settings → Orders →  Purchase Order Approval

You can set the minimum purchase amount limit by Manager or Admin.

Now Create Purchase Order by a User account (Demo User) with an amount exceeding the limit set by Manager or Admin user. 

We have created a RFQ with an amount exceeded so It will go to the To Approve Stage.

Now, Approval Requests will go to Manager/Admin so it can be approved by Manager/Admin. Approve Stage.

Once It is Approved by the Manager/Admin, Now you can see in the Demo User account that it is approved and rest of the procedure will be same as normal purchases.