If one receives a quote from a builder say for €105k is it ok to ask the builder would they do the work for €100k without compromising on the quality of work.
Perhaps. If I said the other two quotes were slightly in excess of €100k but that he who quoted the €105k would be preferred choice would that wash I wonder.
It's perfectly ok to ask for a discount but unless the builder is a fool, they will be highly unlikely to agree if there is any possibility that doing so would compromise the quality of the finished job.
If they do agree, it's probably because they have already built a discount into the quoted price.
Before asking, just make sure beyond doubt that they're not a fool.