Can variables be declared in a conditional formula?

Can variables be declared in a conditional formula?

For example, how would I get something like this to work?

InvoiceNumbers = (Payment[ID > 0].InvoiceNum).getall();
if( InvoiceNumbers .contains(ThisFormInvoiceNumber) == true, ThisFormInvoiceNumber , ThisFormInvoiceNumber  + "<font color=#FF0000>An Invoice Needs to be Made for This Group of Records</font>")