Current behaviour is -

If there is no batch this field is left blank.

If the product has a current batch - the field is hard set to that expiry date.

If this occurs the field cannot be changed by the end user.

There is actually only a few cases were this is factually correct. 

Firstly by law all prescribed medication must be prescribed with some defined expiry date. 

So leaving the field empty is a recipe for disaster.  In my opinion every dispensing should have a soft limit set at record creation if there is NOT an applicable batch in the system that can be applied.  This could be set similarly to the prescription soft expiry.

That being said some medications will have a very short expiry once dispensed that doesnt correspond at all to any batch number  once a product has been opened it must be used in a short expiry fashion

examples are things like Clavulox Drops, Ocuflox Drops, Ear medications

The product oxidises once opened and will expire in under 30 days in some cases. If these products have a batch this prevents the enduser applying these expiry dates manually.

Solution: potentially have an optional expiry period in the medication details that soft sets it self on creation. Ie Clavulox drops have a 2week expiry date after which they should be discarded. So in the medication screen a soft expiry of 14d is set.

This soft expiry can be adjusted upward by the end user at there discretion BUT if the same medication has a batch applied the upward limit of the expiry is constrained by the batch expiry