My car died and I took it to my (frum) mechanic. He replaced some part or other, then discovered another part that needed fixing. He fixed that, then discovered that the head gasket is blown, meaning it would cost $1600 to fix. He did $400 worth of work so far. If I decide to junk the car and forget about it, do I still owe him $400 for what he did?
Although you ultimately chose not to fix the car, you do owe him the $400 for the work he had already done.
The work that was done was ordered, and done properly, and payment is therefore due. For instance, it is clear that if a mechanic fixes a car, and later the car gets blown away by a storm, the work must still be paid for.
If the mechanic was in some way negligent in not finding the major flaw earlier, a claim can be made that he didn’t do his job properly, and the halachah might then change. But assuming he was not at fault, the $400 must be paid.
Best wishes and sorry for the delay in replying.