20 CFR § 404.999d - When and how to claim reimbursement.
(1) Generally, you will be reimbursed for your expenses after your trip. However, travel advances may be authorized if you request prepayment and show that the requested advance is reasonable and necessary.
(3) Arrangements for special means of transportation and related unusual costs may be made only if we or the State agency authorizes the costs in writing in advance of travel, unless the costs are unexpected or unavoidable. If they are unexpected or unavoidable we or the State agency must determine their reasonableness and necessity and must approve them before payment may be made.
(4) If you receive prepayment, you must, within 20 days after your trip, provide to us or the State agency, as appropriate, an itemized list of your actual travel costs and submit supporting receipts. We or the State agency will require you to pay back any balance of the advanced amount that exceeds any approved travel expenses within 20 days after you are notified of the amount of that balance. (State agencies may have their own time limits in place of the 20-day periods in the preceding two sentences.)
(b)You may claim reimbursable travel expenses incurred by your representative for which you have been billed by your representative, except that if your representative makes a claim for them to us or the State, he or she will be reimbursed directly.