I recently came upon the following problem and I thought I would ask you what you feel is the proper halachik choice. I was hired to fix a malfunctioning front door lock. Before seeing the lock I told the home owner that depending on what was wrong I might have to replace the lock at that could cost around $100. They said if that is what is needed then that is fine and of course they would pay for the new lock. They had no literature on the lock or any warranty information. As it turned out a major part needed to be replaced and it was not available separately. A new lock was needed. After going to a local Home Depot I found the matching lock, but I also noticed that this particular lock had a lifetime warranty. Unbeknownst to the home owner I was able to exchange the old lock for a new one at no cost. Actually I bought a new lock but got a refund when I returned the old one. My question is whether I have to pass the savings from the warranty onto the owner or can I tell the owner that they must reimburse me for a new lock?
Yes, you should give the savings to the homeowner. Of course you can claim payment for the work you did.
The reason for this is that once you see that there is a lifetime warranty on the lock, the obligation of “returning lost property” applies, which is pertinent for any case of saving another from financial loss.