They probably lots the records in a move, or the admin bunny who works there cannot find a record. No visible proof means it did not happen, they will not admit to losing anything, FACE.
You have a record of at least having paid, but that is history, you are concerned about the now.

As for the free semester, it is your word against theirs. You can't win that one. The person who made the offer has either left, or been forced by their boss to recant. FACE.

If you do not continue study they are legally obliged to cancel the visa. I am sure that you don't want to give them money to continue study, but they will not give you free tuition.

What you could do is find another school that is willing to give you a study visa. I doubt you will be blacklisted as this is a dispute between customer and vendor.
I don't know how you are talking to them, but I would use email. That way there is a paper trail. Telephone calls are deniable.

In addition to what tigertiger said, always, and I mean ALWAYS, get a paper document, signed and stamped with the school's official stamp, to guarantee ANY promise made, especially something as important as a semester's tuition (anywhere from 3000 to 6500 yuan, which for most of us is a considerable sum of money).

A verbal agreement is worth jack shit.