How to set up a rule so that the memo line becomes mandatory only for specific transaction sources

Summary:

I have requirement to make usage of memo lines mandatory for all AR invoices entered manually. So I want to personalise form to make 'Memo Line' mandatory on AR transaction form for transaction source 'manual'

I was able to personalise form using expression #{bindings.BatchSource.inputValue=='Manual'}. However

When Creates New Transaction By default System shows 10 Lines and user will enter data in 1-2 Lines and Remaining Lines are not required.

System shows Error Message stating memo line is Required and now allowing to complete the transaction.

So have to Manually Delete Addition Lines Before Completing invoice.

So how to deal with this situation. I though to put additional condition to check quantity as well #{bindings.BatchSource.inputValue=='Manual'} and #{bindings.TotalQty.inputValue!=null} but this expression isn't working