AX 2012, expense policy- set up condition

Hi guys,

I have 2 expense categories, Expense Type 1 and Expense Type 2. How can i make a field compulsory for Expense Type 1 but not for Expense Type 2?

For example, when Type 1 is selected, receipt number must be filled up, or error will occur when submitting the report, but this rule doesn't apply to Expense Type 2.

Any suggestion?