It is suggested that Trade agreement ID be added to the Trade agreement Tab in the Price details selection on a sales order. You can personalise but no field exists for the Trade agreement id.

Business dont have a mechanism to identify the original trade agreement id that has been used to establish a price. You could have multiple trade agreements for the same customer.

You can make use of the Find prices option under Sales and marketing > Prices and discounts > Find prices. This will list multiple prices, but doesnt indicate the trade agreement id. You can personalise but no field exists for the Trade agreement id.