Is it permissible to buy an engagement ring for the bride with the association's money before the association's term ends?

If the "association" (al-jam'iyyah) is what people commonly understand today as some employees collecting sums from their salaries and then paying them to one of them, and so on, until it circulates among all of them, then there is no harm in that. A number of contemporary scholars have issued fatwas permitting this, among them Sheikh Abdul Aziz bin Baz, may Allah have mercy on him.

Therefore, it is permissible for you to purchase what you need from your necessities with what you obtain from this association, including buying the "shabka" (engagement gift). The fact that the association's duration has not expired has no bearing, because what is taken from this association is akin to a benevolent loan (qard hasan) without interest.