Final answer:
Seller Thomas's alteration of the original offer's terms is known as a counteroffer. It shows his willingness to negotiate but under different conditions than initially presented by the buyer.
Step-by-step explanation:
When Seller Thomas changed one of the terms of the buyer's offer on his home, this is best described as a counteroffer. A counteroffer is a response to an offer in which the original terms are changed, essentially rejecting the initial offer and presenting a new one. This action indicates that while Seller Thomas is interested in selling, he is not fully satisfied with the terms of the original offer and wishes to negotiate by proposing changes which must then be accepted by the buyer for the sale to proceed.