I need to void an Invoice which is posted. It is a memo invoice whose cost was entered against a venture. The invoice is for testing fees, so there really aren’t goods to return to the warehouse, how do I void the invoice?

Take a look at the invoice and see how you entered it, do it the exact same way with a negative amount. If you look at the original invoice at the top you will see one of two things Memo invoice (customer) or (shipment). If it is (customer) note the misc charge used, the amount and the customer. Then go to Add New Invoice, select that customer and proceed.  If it is (shipment) note the misc charge used, the amount and the shipment, then go to Add New Invoice, select that shipment and proceed. Then select the misc charge and enter the negative amount. Posting this invoice will negate the original invoice