Mechanics lien to be filed on my home

Had historic written quotes from a contractor of $2500 for porch and $3500 for under the house. Earlier this month we had the contractor out to give us current quote to include a little work on the garage in addition to the work on the 2 historic quotes. He quoted a total of $7000 for all 3. He completed the garage but told use the work under the house he was unable to do and that he expected us to pay $6500 for the porch and the garage. The porch is still not complete and he now wants $4700 for the work he has done and that amount does not include any further work. We have offered $3500 as a very fair amount for the work that has been done (even though not complete). He is threatening a lien. What are our options other than of course paying the full $7000.