Custom Field

[gofer123]

Hi,
Bit of a tricky one,
I need to create a custom field.
This field will be a numeric field which will contain either Total Technical Fees Paid By Customer for Rebate  supplied by the Ostendo system (from which the web site will then calculate and display the total available rebate amount based on the rebate rules given)  OR the Actual Customer Rebate Amount (if Ostendo calculates the rebate amount)
Would be grateful for someome's help here.

My setup
VirtueMart 2.6.10
Joomla! 2.5.24
PHP 5.3.10

Cheers