my 2 cents.
5k out of rm150k is no big deal. as another bro mention, 3.3%.
besides this, you did mentioned you have requested him to do multiple things. in which one of it included a freaking staircase. am not sure how much an additional staircase would cost but i would have presume it cost quite a fair bit.
tell him to finish the task at hand. tell him you are going to hold the rm5k balance further to check for defect. i did this to all my contractor. a 6 months liability period before i release full amount.
on the additional sum requested, you need to be objective about it. did you add a whole lot of task or requested for a lot of rework? eg. even though bad workmanship, did you pointed out at him during the process or you wait till it's completed and requested for a rework? it makes a lot of difference.
am not saying the contractor is right but if he really did a good enough job for me and did extra of what i requested, i would do a evaluation and top off a little. in fact i think you are lucky that he only requested an additional rm5k.
and before you repeat the statement of you HATE people who do not stick to their word, please bear in mind you also 'lied' to him about the scope of works. you added a whole lot of works and requested it to be done for free.

omg!
after some time, can't believe this thread still on going.
was amused that such a thread have its sustainability so i decided to revisit.
and omg, still arguing about the 'freebies'.

lesson learnt.
my reno starting next month, am going to ask my contractor for a free house.