Supplier sourcing response allow for negative number

Does current functionality allow for a supplier responding to a RFQ put in a negative number on a line item?  One example is on some construction projects there will be need for deductive alternates line items in which the supplier will need to put a credit for taking some portion of the work out of their bid or for replacing some product with a lesser quality product on individual line items.  Another example could a line item for a particular item and there is a line item for a credit for a trade in.