Hi Tushar Paul,
The way you are expecting "Revenue" type is to be validated with such error message same like "Expense" type, is not possible to achieve in standard application.
At the same time, I will not recommend to go for customization as I am not sure what kind of complexity that may create in ERP.
As mentioned earlier, nobody wants to restrict their revenue generation. So, from that perspective, revenue is not getting validated through budgeting the way it is happening in case of expenses.
Better you can explain this logic to your customer why the functionality is not yet built even though the account type selection option in available in budget register entries form.
Else, you can approach to Microsoft to get a clarity on this further. As per my knowledge, Microsoft do not have any such plan to fix it in any upcoming version as of now. They may ask you to add this in Idea which will be depending on number of voting in order to take a decision on this in future if it becomes a global need!
Regards,
Sourav