how can we configure budget on sales order and purchase order in business central?
your statement is a bit vague as to where do you expect to see the budget. BC doesn't have feature to include budgeted number into a sales order but, per previous response, can set up budgets as daily, monthly, etc. those could be simply created as benchmark to measure the actual transactions versus budgeted target. Here is a link for more detailed reference.
how can we configure budget on sales order and purchase order in business central?
There is definitely standard functionality for this. In Business Central search for Sales Budget and Purchase Budget. You can then use these budgets in analysis reports.