If a reimbursement or refund transaction is received within the same month as the original transaction, track it to the same budget item. Doing this will cause the Remaining column amount for the budget item to increase.
If a reimbursement or refund transaction is received in a different month than the original transaction, track it as income under the Income budget group. This way, you can allocate the reimbursement or refund money to any budget item(s) you choose in your budget.