Yes it can be in another currency. As long as it is equal to 20,000THB. However, that is what is supposed to be allowed. Whether or not it actually is ... well... that is another thing altogether. Welcome to Thailand. But it is also all in your appearance if you are asked. In my 12 years here and my two children (in their 20's) coming and going as well as many other friends that have visited me over the years... I have never had an instant of any of them being asked for proof of funds. It is a crapshoot like most things here with regards to Immagration officials.
See an agent. They will deal with it. As you do have the full amount on a Thai bank they will deal with it. It will cost you a fee for them... but that will be far less than starting all over again. Heard this issue many times before from others who had no difficulties at all this way.
ahh.... just go to an agent.. try TVC here in this group. They have good and real reviews. Save your money. The TEV is fine but if you cannot pay there and then ... you are right.... they don't give a dam.