Arbitration — The law applied by arbitral tribunals — Claim for amount of dishonoured cheques — Whether claimant beneficiary of cheques — International Monetary Fund Agreement — Article VIII, Section 2(b) — Prohibition on enforcement of transactions contrary to exchange controls of another member State — Whether applicable to international arbitral tribunal — Iran–United States Claims Tribunal — Validity of purported currency exchange regulations — Doubts as to nature of underlying transaction — Burden of proof — Whether shifting to claimant when respondent has raised doubts — Plea of unjust enrichment at post-hearing stage — Whether admissible — Whether principle applicable in international arbitral tribunal — The law and practice of the Iran-United States Claims Tribunal