What party must an ODFI of a Same Day ACH enter into an agreement with before submitting ACH entries?

The correct choice is Originator. The ODFI (Originating Depository Financial Institution) must enter into an agreement with the Originator before submitting ACH entries. This agreement ensures that the Originator has the authority to initiate ACH transactions on behalf of their customers and complies with relevant NACHA operating rules and regulations.

The relationship between the ODFI and the Originator is fundamental in the ACH network, as it establishes the legal basis for the transactions. The Originator is responsible for providing the ODFI with correct and valid payment instructions and must also ensure that they have obtained proper authorization from the receiver (the ultimate consumer or business) for the transaction to occur.

Other choices, such as Transaction partner, RDFI, and Custodian bank, do not play a foundational role in the agreement necessary for submitting ACH entries. The Transaction partner may refer to another financial entity involved in the payment process but does not represent the primary relationship needed for ACH submissions. The RDFI is the institution that receives entries; while they are important in the ACH process, their agreement with the ODFI is not a prerequisite for submission. A Custodian bank generally pertains to investment accounts and asset management rather than the ACH framework, thus is not relevant in this context.