So, for Ruby, based on Ywain's answer, I decided that instead of what is documented :
Stripe.api_key = CONNECTED_STRIPE_ACCOUNT_SK Stripe::Balance.retrieve
The best way that is not documented is as follows:
Stripe::Balance.retrieve(stripe_account: CONNECTED_STRIPE_ACCOUNT_ID)
while the current api_key is your platform account with the managed accounts option enabled.
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