Strictly speaking the negative equity isn't the difference between 330k and 250k; it actually depends on how much is outstanding on the mortgage. If for arguments sake the mortgage balance is now 300k, then the buyer is taking on an additional 150k of debt to acquire 125k worth of property. (ignoring additional legal costs etc)
There is no gift tax issue, provided the two parties are unrelated, because it is a de facto arms length transaction, and clearly the arrangement is not being undertaken for the purpose of tax avoidance. (In fact it facilitates the vendor in purchasing a property elsewhere, and paying stamp duty as he will no longer be a FTB!)