Another day, another shonky contractor

[Nat C.]

I have been using Thumbtack to find various professional for hire. Thumbtack is similar to Angies List and Home Advisor.

I placed an ad for a HVAC technician. I offered $250 for installation on each AC unit stating that I needed two units installed in two properties immediately and another two units installed next month. 

I had several offers to complete the work and ended up choosing a company that were close by. 

They did not turn up for the scheduled Tuesday appointment and they rescheduled for Wednesday. The tenants were arriving Wednesday night so it was certainly cutting it fine. On Wednesday the AC guy called me and when they were at the job to tell me it would cost an extra $250 due to extra work and materials they needed to buy. I wasn't in a position to say no, so I agreed.

They finished the job in the afternoon and text me pictures and said they emailed me the invoice.

The tenants arrived later that day and told me the AC unit wasn't working. I then looked at the invoice and saw it is for $1410. I questioned the the company on the additional $910 that I hadn't agreed to pay or been advised of and he said it was extra materials and labor.

To make it look even more like they are trying to pull the wool over my eyes, he said the quote of $250 per unit means $250 for air handler and $250 for the condenser. 

Presently I have a HVAC system that isn't cooling. He said they will come and look at it after I pay the $1410. At no point will I pay $1410, I will only pay $500 because that's what I agreed to. He said he will be placing a mechanics lien on the property. 

I did some reading on mechanics liens and I imagine I would have a chance to contest it and I have thumbtack correspondence in writing that the agreed price is $250 per unit. As mentioned he is claiming per unit pertains to air handler as one unit and condenser as second unit. 

How would my Bigger Pockets friends proceed?