if the user entered an amount less that the money he has in his wallet,can I say you don't have enough money try x$ or it's a bad user experience?
I'd say it's perfectly reasonable to tell the user why it is invalid. Etc. insufficient funds, but I don't believe that necessarily believe you should give an example of what a valid input would be.
Depending on the user group and how good they grasp monetary values, extra instruction might be beneficiary.
It depends. See the possible consequences for these two situations:
The user enters their email while logging in as "email@example.com". It's pretty obviously a typo, so after making sure it was an error, we could say "Maybe you meant 'firstname.lastname@example.org'?". That way we most likely will help them fix their mistake faster. And if it wasn't a mistake - nothing bad will happen, they still won't be able to login, no real damage done.
The user tries to set up their new password, we figure out it's not strong enough, so instead of just stating that, we suggest what we think are better password options for them. If they agree with our suggestion, there's a whole bunch of different security and user risks:
- somebody else (us) knows their password, which is never a good thing;
- knowing our suggestion algorithm makes their password more vulnerable;
- using the password they didn't create on their own might make them forget the password faster;
- in the case the account gets hacked into, we are to blame for suggesting the password that wasn't secure enough;
As you can see, in the second scenario, the cost of bad suggestion is much bigger than in the first one. And we always need to be aware that our suggestions might be wrong.
Now, knowing that, we have to figure out what's the cost of a bad suggestion:
- Can the users accidentally send the wrong amount because of our suggestion?
- Can they misunderstand what we're suggesting?
- Can they freak out about us knowing what their precise amount is?
- Can we leak their private information this way? For example, if they don't expect there to be any info about their balance, could there be somebody else standing by them while sending the money so we accidentally show more than intended?
I'd advice against it - when dealing with money people tend to be very touchy, so we should be very careful with that information.