I am trying to create a field that deducts the "quoted amount" from the "transaction amount" only when the custom object "vendor" Vendor compnay is is Prominent Tickets.  Can someone assist me?

Quoted_Amount__c - Transaction_Amount__c && Vendor__r.Name = Prominent Tickets

I am trying to create to ACCOUNTS within my CRM that use EMAIL TO CASE and allow me to use the SAME email address for both.  The system is not allowing to create both.  Any advice?
 

I am trying to create a field that deducts the "quoted amount" from the "transaction amount" only when the custom object "vendor" Vendor compnay is is Prominent Tickets.  Can someone assist me?

Quoted_Amount__c - Transaction_Amount__c && Vendor__r.Name = Prominent Tickets