Instrumental - when "z" is necessary?

I thought I understand when "z" is necessary but now I think no. Please help.

Mama smaruje chleb masłem. - Masło is tool, so no need to use "z".
Ona lubi chleb z masłem. - Chleb and masło is together, so we speak about two things which are together. I thought this is when we need z.

Other examples:

Zawsze piszę żółtą kredą na tablicy - żółta kreda is tool, no "z"
Kroil mieso nożem - tool, no "z"

Mamy kłopoty z pieniędzmi - why is it "z" here??

I didn't find the rule. Thank you!

Mam kłopoty z pieniędzmi. "Pieniądze" doesn't seem to be a tool here.
Zapłaciłem pieniędzmi (gotówką), a nie kartą. "Pieniądze" does seem to be a tool here.

Besides, there is the verb "have" in your example. Notice that you also say: Mam kłopoty ze swoimi dziećmi, and "dzieci" isn't a tool in this sentence.

Some Polish verbs govern the instrumetnal case. There we use it without the prefix 'z'. English must always use its 'with'.
All activities performed with the use of some instrument, tool or implement take the instrumental case.
Wybił szybę cegłą. (He broke the window with a brick.)
Napisał list długopisem (but na komputerze). He wrote the letter with a ballpen.
Nie jedz palcami! (Don't eat with your fingers!)
Głową muru nie przebijesz! (You won't break diown a wall with your head!)
It's worth memorising some of the more important verbs that require the instrumentall. These incldue:
władać, interesować się, jechać (means of transport in instrumental), fascynować się, etc.

Thank you, you are right pieniędzmi is not a tool here, I don't know why it was strange for me..
Polonius3: thanks, yes I know that there are verbs which we need to use with instrumental. I didn't know that in this case we do not use "z", it is a new information.

Thanks a lot!