Using Document Currency in Custom Field

Dear Friends

If any body can help me on this.

My Company Currency is AED , however , I send quotations in Different currency for exports. I want to add the Custom Field to track some information about the costing of the quotation. Say In a quotation of US$ 5000 , I want to put US$ 4000 as my cost in Custom Field. I have added custom field with currency , but when ever i am adding 4000 , in custom field , it is taking 4000 as AED , not 4000 as USD. I want to use Document Currency for the field , not the company currency. If any body can help me.