Riddle

[Elsie87]

John had just received money as his birthday present from his friends. John then put it inside one of his folders. He happened to know the amount inside without opening the envelope. A few days later, he opened the envelope and found out some money were missing. The envelope, before John opened, was still in sealed condition, no trace of being re-sealed. He then asked David, the one that gave him the envelope, to confirm what he had remembered is true. David confirmed the actual amount. John then told David that the amount didnt match with the one in the envelope and asked how could this happen. David replied he didnt know and asked whether John has spent some of it. John said he didnt spend anything as he just opened the envelope. John asked David if some may have not been put inside or fallen down. David said he had re-counted the money before it was put inside the envelope. He also said he didnt reduce/take the money.
Question : How can the money be lost?
The culprit is not limited to the characters mentioned in the question.

[spathiphyllum]

David is just one of the friends that gave john birthday money.

That's something to take into account

The money went into folders, that's only mentioned once.. Maybe something to note?

Did all the money go into the same place? IE in envelope then in folder?

How many envelopes were there?

[Elsie87]

Actually, David put all the money in an envelope and gave the envelope to John. John then kept the envelope inside his folder. So, in total, there is only one envelope. The one that handed the envelope directly to John is David only.

[Tilly2]

Someone has switched the envelopes whilst John was not looking?

[elliemay1]

Someone in John's family/friends took the envelope and replaced it with an identical one but with less money??

[Calibax]

When David gave the envelope to John it was unsealed. The culprit sealed it after taking the money which is why it did nor appear REsealed.

[Tilly2]

Snap, Elliemay!

[megfitz]

John had more than one folder, he opened a different one.

[Jackdaw33]

The trouble with lateral-thinking questions like these is that there are several plausible answers. It boils down to guessing what the questioner is thinking.

[Tilly2]

Are we going to get any further with this?

[mallyh]

there were 2 johns ?

[Canary42]

Inflation

[mallyh]

Elsie are you going to tell us the answer ?

[Butterbun]

Some cash, the rest a cheque?

[Old_Geezer]

Did they notice the hole in the bottom of the envelope ?

[Tilly2]

Uncle Tom Cobley and his gang took it!

[Elsie87]

Elliemay1's answer could be it. But
What if the envelope is different from the usual one? What I mean is the envelope is not sold in any local store.
It is a special envelope and it is sealed with special tape, e.g. washi tape.

Calibax's answer is plausible too. But
What if John received the envelope in sealed condition directly.

I would like to add some lines to the questions again.
The reason why John had known the amount is because David counted the money in front of John. John only looked at how David counted the money using calculator, but John didnt actually count in his head. John only saw the number appeared in the calculator, which is the total. After David had counted, David put the money inside the special envelope and sealed it with special tape in front of John.

The envelope is only one and it is the only envelope in John's folder.