Assumption here: You are making a many2one field from the product.product to your res.partner object? If so, on the sale order you will need to reach a level up.

When creating new sale order lines, the domain on the product field should have something to do with the order_id.partner_id on the line itself. The line is connected to the order by means of an order_id. 

If that id is unavailable to you at the time of creating, you can always check the on_change method for the product on sale.order.line. This will give you a clue as to how the order line will get the right price (calculating in the pricelist on the sale order). 

If this does not help you, please post your code thus far and we will see what can be done.

In Sale Order added field  customer product ID dyamically based on onchange customer.

class sale_order(osv.osv):
    _inherit = "sale.order"
    
    _columns = {        
        'custproduct_id': fields.char('Customer Product', size=64),
    }
    
    def onchange_partner_id(self, cr, uid, ids, part, context=None):
        res = super(sale_order, self).onchange_partner_id(cr, uid, ids, part, context=context)
        if part:
            product_obj = self.pool.get('product.product')
            matching_cust_ids = product_obj.search(cr, uid, [('x_cust_name','=',part)])            
            res['value'].update({'custproduct_id': matching_cust_ids})            
        return res
        
sale_order()

In Sale order line Form/Tree view inherit to add domain

 <field name="product_id" domain="[('id','in',parent.custproduct_id)]"/>

EDIT: Added my own code written after prakash's comment.

Here is the code I used to put the customer name on the product form, like you said, it's a many2one on product.product:

class ec_products(osv.Model):

    _inherit = 'product.product'

    _columns = {

       'x_cust_name': fields.many2one('res.partner', 'Customer')

       }

I added the entirety of product_id_change() from sale.order.line to my custom module, and added these lines of code:

        matching_cust_ids = product_obj.search(cr, uid, ['x_cust_name','=', partner_id])

        domain = {

        'product_id':[('id', 'in', matching_cust_ids)]

        }

From what I can tell this would scan all the product objects and pick the ones whose 'x_cust_name' field is the same as partner_id, which is passed to the on_change function. Now I am getting an error when I try to add any sales order lines:

File "/usr/lib/pymodules/python2.7/openerp/osv/orm.py", line 4680, in _where_calc if not any(item[0] == 'active' for item in domain): File "/usr/lib/pymodules/python2.7/openerp/osv/orm.py", line 4680, in <genexpr> if not any(item[0] == 'active' for item in domain): TypeError: 'int' object has no attribute '__getitem__'

I know there is something wrong with my code, but I can't figure out what it is. Does self.pool.get('product.product') return a list of all products? I think there is something big I am missing here. Thanks!

Edit 2 - This is where I am adding the domain... is this right?

    <record id="view_res_sale_cust_domain" model="ir.ui.view">

      <field name="name">res.sale.cust.domain</field>

      <field name="inherit_id" ref="sale.view_order_line_tree"/>

      <field name="model">sale.order.line</field>

      <field name="arch" type="xml">

         <field name="product_id" domain="[('id','in',parent.custproduct_id)]"/>

      </field>

    </record>