Standard Price defaulting in PR

Hi There,

We are using PeopleSoft 9.1, When creating a Purchase Requisition prior to entering the Vendor number the price we entered gets overridden with the standard price. So I created a PR then entered the item and the price I desired $500 (manually)...Then I entered the vendor number and I got a message:

Vendor item price is not available - use item standard price.  Line 1, Item 28200045 (10200,197)

The vendor item price was not setup, or the corresponding UOM

doesn't meet the minimum requirements. The item standard price is

used instead.

I click on OK button then the price changed to the standard price  $253.80... Is there a way I can prevent the price from changing to standard price? Please advise.