How to make IExpense attachment mandatory for particular expense?

Hi,

I need to make IExpense attachments mandatory based on line amount. for E.g. if Cash expense is > $25 and credit card expense >$250 i need to make line level attachment mandatory. I tried to modify AP_WEB_CUST_DFLEX_PKG.CustomValidateLine procedure but with this I can make attachments mandatory but not on amount or RECEIPT_REQUIRED_FLAG. Any help is appreciated.