Is it permissible for a member of a financial association, who has paid part of the monthly installments, to use the accumulated amount to go for Umrah in Ramadan, knowing that two installments will remain outstanding on the amount?

There is no objection to this association; it falls under mutual assistance in righteousness and piety. The permissibility of this association has been previously stated on this website. Performing 'Umrah with funds from the association is permissible because the funds used are permissible. There is no problem with outstanding installments for the association, as long as their due date has not arrived, and your performance of 'Umrah does not affect their repayment.