The letter was not sent to your customers for their benefit. It was sent for the funder's.
A UCC-1 financing statement, filed at origination, gives the MCA funder a security interest in your business assets, including accounts receivable. Upon default (or upon the funder's assertion of default, which may not be identical to actual default), the funder may exercise its rights under the lien by contacting your customers and directing them to send payments to the funder rather than to you.
The letter your customer received instructs them to redirect payments. Your customer, understandably confused, calls you. Or worse, complies with the letter without calling.
Assess Whether the Default Has Actually Occurred
The funder may have sent the UCC enforcement notice before a default has technically occurred, or based on a default that is disputed. If you invoked reconciliation and the funder refused, the question of who breached the agreement first may be unresolved. An attorney can evaluate whether the funder's enforcement is premature or unauthorized.
Contact Your Customers Immediately
Do not let the funder's letter be the only communication your customer receives. Speak to each affected customer directly. Explain that the matter is under legal review. Request that payments be held pending resolution. The relationship between you and your customer is your most valuable business asset. The funder's letter threatens that relationship deliberately.
Retain an Attorney to Challenge the UCC Enforcement
A UCC-1 lien is enforceable if it was properly filed and if the underlying agreement supports the scope of the lien. However, the lien may be challengeable if the filing was defective, if the scope exceeds the agreement's terms, if the agreement is reclassifiable as a loan (which may affect the enforceability of the security interest), or if the funder's enforcement is premature.
An attorney can also file an injunction to prevent further customer contact while the dispute is being resolved.