Step-1 Install the Framework_agreement module.
Step-2 Framework_agreement->Model->Purchases.py
Step-3 After that custmozie purchases.py(Python file) file:
In below example, I have custmozied purchases.py file. You can directly copy the below python code in your purchases.py file.

from openerp import models, fields, api
from openerp import exceptions, _


class PurchaseOrder(models.Model):
    _inherit = "purchase.order"

    portfolio_id = fields.Many2one(
        'framework.agreement.portfolio',
        'Portfolio',
        domain="[('supplier_id', '=', partner_id)]",
    )

    @api.onchange('pricelist_id')
    def update_currency_from_pricelist(self):
        """Reproduce the old_api onchange_pricelist from the purchase module.

        We need new-style onchanges to be able to modify agreements on order
        lines, and we cannot have new-style and old-style onchanges at the same
        time.

        """
        self.currency_id = self.pricelist_id.currency_id

    @api.onchange('portfolio_id', 'pricelist_id', 'date_order', 'incoterm_id')
    def update_agreements_in_lines(self):
        Agreement = self.env['framework.agreement']
        if self.portfolio_id:
            for line in self.order_line:
                ag_domain = Agreement.get_agreement_domain(
                    line.product_id.id,
                    line.product_qty,
                    self.portfolio_id.id,
                    self.date_order,
                    self.incoterm_id.id,
                )
                good_agreements = Agreement.search(ag_domain).filtered(
                    lambda a: a.has_currency(self.currency_id))

                if line.framework_agreement_id in good_agreements:
                    pass  # it's good! let's keep it!
                else:
                    if len(good_agreements) == 1:
                        line.framework_agreement_id = good_agreements
                    else:
                        line.framework_agreement_id = Agreement

                if line.framework_agreement_id:
                    line.price_unit = line.framework_agreement_id.get_price(
                        line.product_qty, self.currency_id)
        else:
            self.order_line.write({'framework_agreement_id': False})

    @api.multi
    def onchange_partner_id(self, partner_id):
        """Prevent changes to the supplier if the portfolio is set.

        We use web_context_tunnel in order to keep the original signature.
        """
        res = super(PurchaseOrder, self).onchange_partner_id(partner_id)
        if self.env.context.get('portfolio_id'):
            raise exceptions.Warning(
                _('You cannot change the supplier: '
                  'the PO is linked to an agreement portfolio.')
            )
        return res


class PurchaseOrderLine(models.Model):
    """Add on change on price to raise a warning if line is subject to
    an agreement.
    """