Is it permissible for a maternal cousin to take a share from the association between him and the questioner without the latter's knowledge or consent, on the pretext that the questioner's parents owe his mother?

Your cousin is not entitled to take your money without your consent under the pretext of your parents' debt to his mother. The son is not obligated to pay his father's debt. However, it is desirable for you to pay your father's debt if you are able to do so, in order to absolve him and to show filial piety.