Notwithstanding the first-in, first-out payment application rule described in paragraph (a)(1)(iv)(A) of this section, the taxpayer may apply payments or credits against amounts owed in some other order on its books in accordance with an agreement or understanding of the related parties if the taxpayer can demonstrate that either it or others in its industry, as a regular trade practice, enter into such agreements or understandings in the case of similar balances with unrelated parties ...

(i) Facts. X and Y are members of a group of controlled entities within the meaning of section 482. Assume that the balance of intercompany trade receivables owed by X to Y on June 1 is $100, and that all of the $100 balance represents amounts incurred by X to Y during the month of May. During the month of June X incurs an additional $200 of intercompany trade receivables to Y. Assume that on July 15, $60 is properly credited against X’s intercompany account to Y, and that $240 is properly credited against the intercompany account on August 31. Assume that under paragraph (a)(1)(iii)(B) of this section interest must be charged on X’s intercompany trade receivables to Y beginning with the first day of the third calendar month following the month the intercompany trade receivables arise, and that no alternative interest-free period applies. Thus, the interest-free period for intercompany trade receivables incurred during the month of May ends on July 31, and the interest-free period for intercompany trade receivables incurred during the month of June ends on August 31. (ii) Application of payments. Using a FIFO payment order, the aggregate payments of $300 are applied first to the opening June balance, and then to the additional amounts incurred during the month of June. With respect to X’s June opening balance of $100, no interest is required to be accrued on $60 of such balance paid by X on July 15, because such portion was paid within its interest-free period. Interest for 31 days, from August 1 to August 31 inclusive, is required to be accrued on the $40 portion of the opening balance not paid until August 31. No interest is required to be accrued on the $200 of intercompany trade receivables X incurred to Y during June because the $240 credited on August 31, after eliminating the $40 of indebtedness remaining from periods before June, also eliminated the $200 incurred by X during June prior to the end of the interest-free period for that amount. The amount of interest incurred by X to Y on the $40 amount during August creates bona fide indebtedness between controlled entities and is subject to the provisions of paragraph (a)(1)(iii)(A) of this section without regard to any of the exceptions contained in paragraphs (a)(1)(iii)(B) through (E) ...

Except as otherwise provided in this paragraph (a)(1)(iv), in determining the period of time for which an amount owed by one member of the group to another member is outstanding, payments or other credits to an account are considered to be applied against the earliest amount outstanding, that is, payments or credits are applied against amounts in a first-in, first-out (FIFO) order. Thus, tracing payments to individual intercompany trade receivables is generally not required in order to determine whether a particular intercompany trade receivable has been paid within the applicable interest-free period determined under paragraph (a)(1)(iii) of this section. The application of this paragraph (a)(1)(iv)(A) may be illustrated by the following example: ...

...