Legal Fees Invoiced but Collection Deferred - Prohibited Transaction?

I am more than a bit surprised that there is no message board devoted primarily to ERISA fiduciary and/or prohibited transaction issues. Our client, a multiemployer 401(k) profit sharing plan (Plan A) is contemplating a merger with another multiemployer 401(k) plan (Plan B). Under Plan A, legal services have been provided to and invoiced by the firm providing them. However, the firm has foregone any efforts to enforce their collection. Isn't the law firm's forebearance in collecting its billed fees an extension of credit and therefore a prohibited transaction? If so, is there an applicable class exemption that might exempt it from constituting a prohibited transaction? Thanks!

Prohibited Transaction Exemption 80-26, a class exemption, sets conditions for a service provider’s interest-free loan to a plan.
https://www.dol.gov/agencies/ebsa/laws-and-regulations/rules-and-regulations/exemptions/class