Create parameter validation "not in"/"not exists"

Hi,

I'm trying to find a solution to a parameter for a concurrent request that would validate that the data IS NOT in a table.

The concurrent is a stored PL/SQL procedure. Based on the parameters it will trigger pre_autoinvoice_master and pass it's parameters to it.

I have a Parameter for Invoice number which has no validation at the moment. - so it will allow any char

What I'm trying to achieve is to test first if the invoice number (trx_number) exists in ra_customer_trx_all and not allow the user to enter it if it exists