You need to go settings--> Configuration-->Purchase

then there find " double validation on purchase " check that box and insert the limit

so you wil be able to do this (Double validation for purchase user  )

HI, Fekete Mihai  i am using purchase_double_validation module that will check purchase user permission person only validated but i want to give validation for purchase manager also, could you help me.

You can use the purchase_double_validation module, and from purchase settings you are allowed to set a purchase order limit, which will modify the purchase workflow adding a new step, validation from purchase manager when the purchase total amount will exceed that limit.