Translation:I only have 800 Yuan.
I have omited "GOT" - and DL marked it "wrong". As I get known from the other discussions - here is a problem between British English and American English. But it should be consistent - in many cases the American English (without GOT) is prefered, so why not here? I have reported it.
快 is a classifier for money, so technically all currency should work, but since this is a chinese lessons, having other currencies shouldn't be neccesary, it would be a big mistake to allow dollars or pound, because theyn you have to add all currencies as well, which is too many. But please at least allow "rmb", "renminbi" and these...
Even though it's a Chinese lesson, here's an English one:
The adverb 'only' stresses the noun or verb you place it with.
"I only have 800 yuan" - you are the single person who has 800 yuan, no one else
"I have only 800 yuan - you have this amount of money and no more
It can also stress a phrase, but this is in context. In written form, especially in examples, placement matters.
Your intention is correct, but your first example is incorrect. In English, you'd have to say "Only I have 800 Yuan" if you meant "you are the single person who has 800 Yuan, no one else." Your two examples both would be interpreted the same way, the meaning "I have this amount of money but no more."