Happened to me earlier this year. Bought a condo with known leaking problem. Then I used that no bring down the house price to ~20K below market price.
Lucky for me the owner upstairs cooperate with me.
Apparently there are few ways to repair the leaking
Mine used the "fluid cement" solution which they basically flooded the toilet upstairs with this whitish milky solution which then sealed the leaking.
Works well.
No leaking since then.
Total cost RM 4.5K, i paid 2.5K.

I already open a new thread at renovation session there but its gone. I guess discuss it here is good enough. I think RM400 is enough when i let them see, they are flexible since there are contract labors.

But there is another simple way traditional way of solving it which is create a drainage direct it away. That's the simplest way to solve with minimum cost only. No need to spend so much money in it. Not sure if your contractor willing to do it for you.

Since your case is very minimum, there is no need to hack everything and redo. Too costly for that minimum leaking.

Call mr wan @ 016 336 1104 (tell him you are from lowyat)