I can understand such requirements and had them too in the past.
However, those requirements may end in screwing up your picking strategy, will cause countless batches with small leftovers and finally dead stock which has to be scrapped because the shelf life date is exhausted.
I had once a case the customer always wanted just 1 batch. Our batch size was 10 tons, he always ordered 8 tons. After a while the warehouse manager complained about those many batches the warehouse and we made an analysis and found that this customer was the only customer for that material. So we asked him to either buy the whole batch at once or to accept a delivery of 2 batches (which should not give him extra analysis costs as he had received this batch already)
Analyze your case in a deeper detail. Find out how big the batch quantity is at production. Find out how much this customer orders in one order. Assume you really use FIFO for all others.
Now calculate on paper or in Excel if you ever could theoretically end up with more than 2 batches for this customer.
Most probably the system just suffers on a wrong pick from the past which caused that you have more than 2 batches of one material in stock. Or you fulfill so many customer wishes that you end up in countless smaller left over quantities per batch.