I've never come across the phrase "niet schrapen maar laten" used like this. It is definitely not a well known idiom in Dutch.

Schrapen is probably related to these senses (from Van Dale):

schrapen:
fi­guur­lijk bij klei­ne beet­jes bij­een­bren­gen, m.n. uit heb­zucht
• geld bij el­kaar schra­pen
• ra­pen en schra­pen

and/or:

de pot uit­schra­pen

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or Juridisch-Economisch Lexicon:

schraper =
(gierig persoon) money-grubber

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Oxforddictionaries.com:

• money-grubbing
(also money-grabbing)
Overeager to make money; grasping.

SYNONYMS:
acquisitive, avaricious, grasping, money-grabbing, greedy, rapacious, mercenary, materialistic
North American informal grabby


• grasping:
Avaricious, greedy

SYNONYMS:
avaricious, acquisitive, greedy, rapacious, grabbing, usurious, covetous, venal;
mercenary, materialistic, money-oriented;

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re "onder aan de streep"
I'm assuming this refers to this (lifted from Van Dale):

"(figuurlijk) hoeveel blijft er onder (aan) de streep over?
how much money will it bring in, what will be the profits"

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I think I prefer using the term "greed(y)" instead of "money-grubbing", so, putting all this together, I ended up with something like:

Het uitgangspunt 'niet schrapen maar laten' werkt alleen als je respecteert dat de ander ook nog iets over mag houden onder aan de streep.

=

The principle of ‘Not being greedy’ only works if you respect the other person’s right to some of the profits at the end of the day.
The principle of ‘Not being greedy’ only works if you respect the other person’s right to a share of the profits at the end of the day.
The principle of ‘Not being greedy’ will only work if you respect the other person’s right to a share of the profits at the end of the day.

"at the end of the day" is optional
maybe change "person" to "party"


see e.g.:

"Alex’s second piece of advice is to not be greedy. He wishes that he had followed this piece of advice when he first started investing in real estate because he has gotten greedy on multiple occasions, which has cost him a lot of business. Alex believes he has made many more mistakes than most investors and greed was the main catalyst. Therefore, adopting a non-greedy principle as one of your business values will help you avoid making many mistakes." (https://www.bestevercre.com/blog/set-goals-dont-be-greedy-cr... )

+

"Not being greedy (to put it negatively) is, perhaps, first and foremost among the business virtues." (https://www.google.co.uk/books/edition/A_Companion_to_Busine... )