Is there a way to reverse M5 transactions (Receipts without purchase order)

There is no way to reverse them. You need to create Issue Inventory part which will create the M2 postings.

However, you may not need to do anything with the part. If acceptable, just fix the balance with the manual voucher. Issue/Receive inventory part transactions are not linked to anything else, so as long as you are not using the counter-postings for any custom reporting that should be fine.

Otherwise you’ll need to consider that these parts might have been already consumed, so after fixing this with M2 you will need to create M5 and M2 again ...and you are in the same place again.

Generally, I would recommend setting M5 and M2 to a transitory / interim account and move this balance accordingly at month end.

Setting up the M110 and M11 posting control will help you fixing it as you will be able to enter the account for M2/M5 manually.