If you are seeking a reimbursement from petty cash, you'll need to complete a Petty Cash Pay-Out Form found on this page:
http://www.northwestern.edu/financial-operations/policies-procedures/purchasing/purchasingpaymentother.html#pettycash and submit it to your department's fund custodian with original detailed receipts. The custodian will make the payout directly to you for any approved requests. The policy for disbursing Petty Cash funds can be found at: http://www.northwestern.edu/financial-operations/controller/treasury-operations/depository-services/cash-policy.pdf