Is it possible to add value date field in the debit entry?

Hi gurus,

I'm an abapper and struggling to be an FI consultant since I have no one to consult with. Here's my issue:

I want to add the Value Date field in a GL acct debit entry. I used optional in Field status group. It doesn't appear unless if I change the posting key to credit. But if I change it to required entry, an error appears " Rules for posting key 40 and acct 115-30-002 set incorrectly for "VALUT" field . Did I miss anything? As much as possible, I don't want to use the other GL Account (a credit entry) since I dont need it in the program flow.

Need an expert explanation on this. Sorry I only have limited knowlege with regards to FI concepts.