Elisa_0593 How are you going into build mode? Are you going from the map screen or are you going there from live mode? As PipMenace said, if you go there from the map, you will always be in free build mode. If you want funds to be limited to the household funds available, you must go there from live mode.
if you are going to build mode from live mode and still get unlimited funds, then it sounds like a mod is doing this. How did you check for a mod causing you to go into free build? Did you run the game without your mods folder in The Sims 4 folder? If it does not happen when you do that, it's something in your mods folder causing it.
Hope this helps.