This is definitely a situation where I would want to slow down and document everything carefully. The main concern is that the refund appears to have been paid to the plan sponsor instead of the plan, which means the plan assets may have temporarily been outside the plan’s trust structure.
Whether this rises to the level of a prohibited transaction depends on the specific facts, including how the advisory fees were structured, whether the fees were actually paid from plan assets, the plan document/fee arrangement, and whether the sponsor or any party received any benefit from the funds being held in the corporate account.
If the refund was intended to return plan assets, the cleanest correction would likely be to return the full amount to the plan as soon as possible, allocate it appropriately to affected participant accounts, and maintain detailed records explaining:
why the refund was issued,
why the check was made payable to the sponsor,
when the funds were received and returned,
how the allocation was calculated.
I would also avoid simply moving the money back without consulting ERISA counsel or the plan’s third-party administrator, since the handling of plan assets and potential prohibited transactions can have reporting and correction implications.
The key question is: were the advisory fees originally paid from plan assets (for example, as a plan expense deducted from participant accounts), or were they paid directly by the sponsor outside the plan? That distinction could significantly change the analysis.