Don't make this decision in a vacum. If it's a $150 fee and the return segments are worth $500, then pay the fee and keep the credit (unless you aren't flying again for the next year). If the return segments are worth less than the fee, let it go.

As a caution should this ever occur again, there was no reason to book yourself on a RT unless it was cheaper than the OW. Even when traveling with a group (4 is not really a group) there is no value. You could have booked 3 RT and 1 OW (for yourself).