Lot availability versus Item availability

MRP considers availability on an item level, but how to handle shortages on a lot number level?

For example one item number, two different lots:

Lot A #50 (oldest lot)

Lot B #75

Two situations:

1) SO demand for #60; lot A will be committed and a second line will be created committing on lot B for #10

2) Demand for #100 (soft commit SO or forecast); MRP has an item availability of #125, so no shortage and no messages are created.

In the first situation the commitment process is splitting over different lots, but needed is commiment on one identical lot number. Although Lot A is the oldest, with a demand of #60 the commitment should be on the next lot with enough availability. In this case lot B and if no lot has enough availability at all - i.e. demand of #100 - the sales order line should go into backorder or be cancelled.