Answer:
Assuming the entry includes a valid account number and account owner name, you have no right to hold the funds or prevent your customer from accessing them. But the ODFI can, within 5 banking days of the Settlement Date, send a reversing entry. If the funds are still in the account, you can honor that reversing entry, but know that your customer may attempt to recover the funds. The ODFI, when it reverses an entry, indemnifies your bank against your customer's claim to the funds (see section 2.9, and particularly subsection 2.9.4 of NACHA Rules).