(1) If any claim or any part of a claim is rejected, the department may, after investigation, require the claimant to file an amended statement or provide supporting documentation before action is taken under the following terms:

(a) Supporting documentation must be provided within 30 days after the request, or the refund request will be denied, and the claimant must re-submit the claim within the time allotted in 15-70-432 MCA; and

(b) If a refund has already been paid and no supporting documentation is submitted upon request, the department will send an invoice to the claimant for repayment.

(2) If the taxpayer submits an amended claim, the claim is reasonable, and the taxpayer has furnished substantial proof, the department in its discretion may accept the amended claim.

(3) A taxpayer may, on the taxpayer's own initiative, file an amended claim. If the claim is reasonable and the taxpayer has furnished substantial proof, the department, in its discretion, may accept the claim.